Don't be afraid to ask the questions that concern you, because I'm sure everyone else is concerned too.
In general, teachers realize how scary it is for freshman and will not be too tough on you at first. The work level will probably be about what it is at your jr. high the first few weeks. Sometimes the jr. high's take orientation trips to see the high school. If yours does, don't be afraid to ask the questions that concern you, because I'm sure everyone else is concerned too. If you don't have an orientation, talk with some of your teachers about what they know about the high school, and what to expect.
There are always the horror stories passed around about how seniors like to terrorize freshman. Usually it's nothing to worry about. They might call "Hey freshman" and laugh as you walk down the hall; but the jokes and pranks usually subside after the first week or so. Again, it differs from school to school, but most of the people I've talked to from different schools around the country haven't had any problems.
I wish you luck, and hope you enjoy your high school years :)