you should get started now if your parents can't contribute as much as you'd like.
a summer job is an option. for example, if your states minimum wage is $7, you'd be making $1120 full time (40 hr/wk) a month and summer is about 2-3 months? that's $3360 (without taxes)
you could take out loans. check collgeboard for that cus i don't knwo too much about loans =( you do have to pay loans back though.
financial aid. contact your schools financial aid department and get as much info as you can. read on their websites and see what they say! for a university i got into that was about $15,000 a month, i got $4,000 financial aid. my parent contribution could have paid for the rest.
scholarships are your best bet. i got lazy and didn't do the scholarships that required essays but you should if you want the money. it'll be worth it in the long shot. apply to as many as you can and do your best with each one. plus, its money you dont have to pay back.
while you're in school, you can do workstudy to pay off your tuition. you'll work on campus while you're in school and the money will go towards paying your tuition.
what i'm doing, though, is going to city college for 2 years for my general education (which is the same at any university, basically). at my city college, its only $700 a year and i'll be working so i'll have money to transfer to a university in 2 years. i'm saving ALOT of money. this could be an option for you, but talk to your parents or a counselor before you make your decision.
also - my counselor told me city college students are more likely to get into a university (than a student applying to a university as a freshman) because they attended city college first.