First of all, Julie, welcome
I'm not as experienced as other people here, but I'll try to give you some help.
1) Find a BR (Beta-Reader). He/She will help you with the grammar and vocabulary, the plotline, if you get stuck, and will warn you if you walk out of character.
2) Start posting here before submitting to the Archive. We all like to read more, and we'll be more than glad to provide you with feedback and constructive criticism, that is, if you want us to
3) Don't be nervous about anything. Fanfic is also funfic - you have to have fun reading and writing it. If you need help with your story, just post here, and be sure that if someone points out something they didn't like in your story, it will only be to help you improve.
I hope to read your story soon.
AnnaBtG.
P.S.: Your favourite writer is Wendy? You have a good taste