Check out
this thread and
this thread for answers to similar questions. Also read through the threads on this and other forums. Ask lots of questions. Think about looking for a bike at the end of the season, when they're sold at clearance prices, Think about getting a lightly ridden, good condition used bike. Check
this site out to figure out the proper size bike for you.
Ignore all the advice to get a fixed gear-- it won't do what you want it to.
Good luck!
EDIT: link fixed.