I was a biochem major and art minor, and I worked about 25-30 hours a week while in undergrad. As a pre-med, I also did volunteer work. I didn't need to work so much, but I wanted to--to be able to afford trips, coffee with friends, etc, on top of rent, groceries, car insurance and gas. The work also kept down the amount of money I had to repay at the end of undergrad. And the work didn't effect my GPA at all, which I know is a big concern for pre-meds--I graduated with a cumulative 3.98 and a pretty decent MCAT score. Actually, I think working gave me a time crunch that made my studying more productive (and you'll hear many students say that their grades went up when they started working).
My advice is to take it slow your first year, though. I started working about 5 hours a week my first semester as a freshman, upped it to 10 the second semester, and then up to 25-30 as a sophomore. You need to figure out what amount of work you can handle, and that first semester is a challenging one academically. You'll know what works best for you. Also, since you have a car, try to find a job that pays good money. I found one that paid $12/hour off campus, but on-campus jobs usually don't pay more than $6/hour. If you can find a medically-related job, that's good for your med school applications, too. (try looking at secretarial jobs on in-patient floors in hospitals. They often let you work one or two days a week, or four hours in the evening, with flexible hours, and the pay is decent.)
Good luck!
