If you have the bookbag you took to school with you, you're set to start. If you don't, buying a Jansport bookbag from Target will get you started for $10-$15. Still follow the recommendation to bring your ten essentials + any gear you'd put in the pack to the store to ensure it'll fit.

After you've bummed around with that for a while, then think about what more you'd like (a hip belt, a hydration bladder pocket, specific photography gear padding, etc).

Gear is supposed to solve problems that you have. If you have no problem, you don't need a new widget to fix the problem.