OP, with an advisor and a copy of the MSAR (Medical School Admissions Requirements, which your college should have a copy of) you can investigate what the schools that you are interested in, need. At most schools, if you major in biology or whatever, you will get all of the requirements done and then some. At the university of california, we have to fulfill some general education requirements, so that takes care of needing things like english composition (You just take a years' worth for your degree), foreign language, etc.
Remember that summer session is your friend if you want to ease into doing well in college. Take 12 units your first quarter (like three classes, with labs). This is plenty if those units are coming from Gen Chem, with Lab, Biology I, with lab, and a math class or whatever. Get A's in all of those. Once you've got a feel for it, then add additional stuff. You can worry about Psychology and Anthropology and Underwater Basketweaving in the summer if you need to take those things.
Ignore the gunners. In real life, they are just a garden-variety a-hole. They will tell you that they can take 20 units of really hard upper division science classes, work a full time job, be an all-star athlete and find the cure for cancer all at once. Oh! And did I mention? They don't have to STUDY either. All of this is BS. Worry about yourself. This is the best advice I can give you. Get good grades in the Biology, Chemistry, Physics and Math requirements and don't be afraid to use your campus's notetaking service, help from TA's/Professors, free tutoring, etc. An A is an A is an A. Okay?
Best of luck to you! You will figure it out.