Hi Megan,
My daughter is also 12 (13 next year) and she was worried when we came over last year about making new friends and fitting in. Well, I can tell you that after her first week at school she didn't look back - she loves her school and her new friends. As AngPhil says, just be yourself and you'll be absolutely fine. In my daughter's class the kids are from all over the world, so they're pretty used to new faces and bringing them into the fold.
We lived in Edinburgh for a long time, but I am actually from Harrogate, N.Yorks So, although my daughter isn't from there she's heard much of the Yorshire accent over the years!
As for gymnatics clubs, I know that there are a few around. Some very good ones too. This link may help you http://www.gymsa.com.au/
The same invitation goes to you. If you would like to email or speak to my daughter, just pm us.
All the best - and good luck!