Step one: enjoy/love/like the show, book, movie, and/or band.
Good job, that's it, you're a true fan.
One thing that just never fails to annoy me is when someone tells me (or another person honestly) that they aren't a true fan of whatever it is they are a fan of for having a certain opinion.
One of the most common things I get is 'you're not a true fan because you like the villains more'. Okay no let's talk about this for a second; the villains are a crucial part of the story. Villains are what essentially gives the show/book/movie its plot. Without villains there is no conflict and without conflict, sure there's a story/plot, but it wouldn't be a very exciting one.
So of course people are going to like the baddies and that's okay. Why? Because villains can be and are well written, interesting, and complex. Why? Because they help make the story fun. I just don't get how one can tell another they aren't a real fan of something for liking a character critical to the story line.
I can't speak for everyone but I personally (as a villain fan) tend to hate the mains. This is another biggy for people pulling the 'you're not a true fan' card.
People it's possible to not like the main character and still like the book or show or whatever. While, yes, the main is a huge huge part of the media in question, he/she isn't all of it. Harry Potter for example; I think he's kind of boring but I still enjoy the books due in part to a fascinating plot and the other characters around him. "I don't like the main" =/= "this book/show is boring". It simply means that the main doesn't appeal to you and that you enjoy the show/book for other reasons. The main character is not everything.
Hand in hand with this I've seen "you're not a true fan if you like this character better than this character. Fandoms are diverse in opinions, of course someone is going to think character A is better than character B and vice versa. I don't know why this is even a cause to tell someone they aren't a true fan with such a vast assortment of characters to choose form. Moreover I usually see this the most when character A is a villain and character B is a hero.
And in the musical case; if you like this singer better than this one. For instance Nightwish; I've seen "you're not a true fan if you prefer Annette or Floor to Tarja (the band's first vocalist). Different vocalists are simply more pleasing to some people's tastes than they are to others. People can't help what vocals are more pleasant to them. It's like trying to choose to become left handed when you're right handed.
In general having a different and/or strange headcanon or perspective of a character than someone else is okay. Different life experiences make way for different ways of seeing the story and the characters in it and how relateable the characters are. That in mind someone can very well see something you don't in a character and the other way around. For example over on the Once spot (way back when spoiler Regina was still evil) I always though she had a lot of good left in her. Person B disagreed...and disregarded my points, telling me I wasn't a true Regina fan for believing she could turn good again and that she truly loved her son. End spoiler. Don't tell someone they aren't a true fan of a character because they view him/her differently from you; for one it's rude, for seconds they're ideals could very well turn out to be true in canon.
How about dubs? Guess what someone can be a fan of something for liking dub X more than dub Y. This is mostly a thing I've seen in the Winx fandom since we have like 3 versions (4kids, Rai, and Nick--Rai being the original). I have gotten so much crap for liking 4kids to the original. I've also seen the reverse. And with anime like FMA that have two versions; you can like the 2003 version more than Brotherhood and the other way around and still be a fan.
Along the same lines (and this is more of an anime thing too) I think people get way too butthurt when someone says "dubbed is better than subbed". You can be a true anime fan and prefer an English dub to the English subtitles. Some people don't like trying to read the subs while trying to take in the actual imagery at the same time. It can be kind of a pain. You don't have to watch the Japanese version of anime to be a true anime fan.
Another thing that gets me every time is when that crazy mega-fan starts freaking out because someone doesn't know every little plot detail, character name, actor name, etc. So what? Are all the people new to a fandom not real fans then? Let me tell you; I couldn't even name all the main and re-occurring characters when I first started watching Naruto (honestly there are so many characters in this one I still don't know all of them). Hell I confused Lydia for another character when I first started watching Teen Wolf. It takes time and getting used to. It takes time to start putting names to faces flawlessly and easily. Eventually you start get the hang of it and it just comes naturally.
And then there are people who simply don't have the time/patience/capacity to memorize line, important date, character name, and what not in the show.
Some people are just casual fans and casual fans are true fans too. And some people simply aren't obsessed with the show/book. Liking something should not be a chore. Your shouldn't have to do a butt ton of research to be recoginzed as a fan.
In the musical sense you don't have to know all the lyrics nor all the track names and or albums. If you like a bad you're a true fan. Nor should you have to know all the band members' names right away--or ever if you're only interested in hearing the music (which is perfectly okay too). You also shouldn't have to be able to say who is who right on the spot. With k-pop bands like SNSD (that have like 9 singers) it's hard to tell who is who right away because they don't have different rolls so you can't just be like; 'oh so and so is the guitarist' you actually have to--like a teacher with new students--get used to which face belongs to which name. Basically it's the same as the above. You don't have to be a super fan to be a real fan.
Much like the preceeder; one doesn't have to have seen every episode/read every book/listened to all the songs on every album. Again, listening to music/watching a show shouldn't be a chore...it's entertainment, something you do for fun. If you have the time to read every book, see every episode, hear every song, more power to you. That's awesome. But not everyone can and that doesn't make them any less of a fan.
The same goes for not watching every interview; be it a band interview or cast interviews. Particularly cast interviews, there's such a vast amount of them, it's hard to keep track.
Shipping is another huge one; someone can ship, ship X and not ship Y and still be a fan. You can prefer ship A to ship B and you're opinion still makes you a genuine fan. I really just don't understand how someone can tell another that they don't truly like a show/book because they like/don't like a certain pairing--especially if the shipping is just a subplot...which it usually is unless you're talking about the romance genre.
Hell you can not ship and all and still be a real fan.
Speaking of likes vs dislikes a person doesn't have to love every. Single. Aspect of a show/book/band to be a real fan. No matter what form of media we're speaking of, it has flaws. There are positives and negatives to everything one likes. You don't have to love every single character, plot, or subplot in your favorite show. You don't have to love every song, lyric, album art, or member of your favorite band. A person can see the flaws in their faves and still be a true fan.
A person is allowed to prefer the adaption to the original as well. Be it you prefer the movie to the book or the tv show to the book or the other way around. You're still a true fan. It doesn't matter if you like the adaptation more than the original nor if you prefer the original to the adaptation. I just don't see the need for people who like the original media more to have such a superiority complex.
Finally you don't need to own all the merch or any at all to be a true fan. Some people just aren't interested in fangear others don't have the money to blow. Likewise with cosplay. Cosplay is expensive shit. Whether you make your own costume or not, it's expensive and sometimes time-consuming.
I lied...this is the finally. To go with cosplay are Cons. A person doesn't have to attend any cons to be a real fan; like cosplay cons are expensive shit (especially if they are out of state or country). Or--for all you HP fans--you don't have to go to the Wizzarding World of HP ever in your life if you don't have the money. You're still a fan, end of story. Excessive spending doesn't make you any more of a fan than the next guy.
And these points are just scratching the surface; these are only a few of the things I've been told and/or seen around the net.
You can totally be and are a true fan of something if you hold one or more of these opinions. Why? Because everyone has a different perspective; everyone has gone through different things and has different knowledge levels and therefore everyone has different points of view of the same show/book/movie/band. Just because someone holds a different view point than your own, doesn't mean they are any less of a fan than you.
To sum up; the notion of a 'true fan' and the very term 'true fan' is bullshit. :)