I went to a catholic school too and I really liked it.
Just be friendly and don't worry too much. You will have friends by the end of the first day. Freinds are the ones who like you because of who you are, not how much money how have or stuff like that. Keep this in mind. It MAY come in handy.
EVERYONE is scared on their first day, the teachers know this and they will be cool.
If anyone decides to pick on you just say
"I'm cool with who I am, If you're not that's your problem"
They can't argue with that.
Bullies are usually just scared and insecure themselves and that's how they deal with it.
It doesn't mean that you are not cool and it's not OK to be you because you are and it is. B)
If you have a problem with bullies (and you may not so don't stress about it) tell a teacher you trust before it gets out of hand. They are trained to deal with it.
Don't be afraid if you get lost on your first day (I did!) There is always someone to help you. Most schools asssign new students an older student as a "buddy" to show you around and help you out for the first week.
Colourcode your time table. You need to be pretty organized in high school to keep track of all your different work. Homework will pick up slowly too (not much at first usually though).
Read Harry Potter and the philosophers stone if you haven't already. That should help!
Most importantly : HAVE FUN!