Well you can download a programme like photoshop or Paint Shop Pro but if you aren't able to do that, there are links on the internet that make banners. The only problem is, these programmes tend to leave there web address quite big at the moment so I think the best thing is to download paint shop pro. (Although you can technically make one on paint which I think comes with most windows systems)
Depends on how fancy you want it to be... If you just want to crop an image into a banner size, then your best bet is Irfanview. If you want to add fancy effects, you're better off with a program like GIMP or Pixia. All three of these programs are free... so it's just a matter of trying them out and seeing which one works for you.
