I want to be a doctor, however I slacked off in high school, do I still have a chance? I'm 18.?

I slacked off throughout high school. I've been on both sides of the fence, I was bullied freshman / sophomore year so I dropped out and attended an online high school. My junior and senior year, I did very stupid things to fit in. I started becoming involved with the "popular" crowd who drank and did drugs, which ultimately led me being the one who did the drugs and drank. I feel horrible for letting myself go down that road, but it happened, I can't change it. I fortunately did get accepted into a four year college, but I don't have the rigorous curriculum background that most students in pursuit of a doctoral degree do. I'm horrible at math, but I'm willing to work. I've finally discovered what I want to do with my life. What can I do to reach my goals, besides the obvious hard work.