Writing original character fanfiction! Advice?

i have a story idea for you. an American exchange student at Hogwarts. now what do you think of that?? also, what is a Mary sue?

I would just say try your best and that's all you can do.

About plot issues: maybe plan it out in advance and insist that you stick to it, instead of adding whatever random ideas that come to you when you're writing.

About Mary Sues: make sure that you give the OC enough weaknesses. And real weaknesses too, ones which make him or her make mistakes, not just cutesy ones. For each OC, list the weaknesses to make sure they're enough. Also, check how you describe him or her. If you're being really flowery about how beautiful he or she is, or how perfect or how much everyone else loves him or her, you'll want to cut that out.