The easiest way to figure out how much you'll need for college is to break all of your expenses down:
Group 1: Big costs. Figure out your major expenses: tuition, fees, housing, food. Even if all of these expenses are covered by financial aid, they're still your expenses to deal with -- so write 'em down.
Group 2: Smaller, recurring costs. These should include the following: utilities (like electricity, gas, cable/internet, cell phone, etc.), commuting/parking, personal (shampoo, laundry soap, razor blades, deodorant, paper towels, etc.).
Group 3: One-time expenses. You may only need funds for these expenses a few times throughout the year, but you'll be in trouble if you don't budget for them. One-time expenses often include books, flights home during holidays/vacations, medical care (seeing the campus health center when you have strep throat, for example), replacement/repairs of lost or damaged electronics.
To figure out how much college will cost in your particular situation, add up all of your expenses:
(Group 1) + (Group 2) + (Group 3) = Cost of College for You
To figure out how much money you'll need, subtract how much money you'll have (from things like scholarships and parental support) from how much your college costs are:
(Cost of College for You) - (Scholarships and Parental Support) = Money You'll Need
Your "Money You'll Need" amount is up to you. You can use savings from a summer job; you can get an on-campus or off-campus job during the school year; you can take out loans; you can do a combination of options like these. No matter how you cover the difference, however, know how much money you'll need during your time in college before you begin your first day of classes.