I loved Middle School, it was a lot of fun.
You do get a lot more freedom, getting to change classes, walking arounf the school, having different teachers, etc. But you still have to walk in lines, no talking in the hallway, and that stuff (at least at my Middle School you had to).
Don't worry about the different classes. That's the best part. Changing classes and teachers each period is fun. Plus, when I was in 6th grade all my classes were next to each other, so I didn't have to worry about getting lost. And your teachers will be more than happy to help you with anything.
Umm... tips... tips... I'd say don't annoy the 8th graders (assuming that your school runs 6th-8th grade). I know the 6th graders always annoyed me last year. Of course now, the 12th graders are going to think I'm annoying, but oh well, haha.