Well I would assume that making friends here is quite similar to where ever it is you came from... As for finding your classes... when I got into middle school as a 6th grader I was scared as hell of the same things you are but you figure stuff out my middle school years were the best years of my life I loved my middle school
So what I'm trying to say is you'll do fine I know a girl who moved here from England like 6 months ago she is really cool and fun to be around.. About the "American-English" thing, all I can say is don't try to be someone you're not. If you meet people who actually want to be your friends they wont care about something as stupid as an accent.