What is required of a student looking to study medicine?

Your major in college does not matter for medical school, in fact they like well rounded individuals, so Liberal Arts would be fine.

However, there are required courses that need to be taken, and a college guidance counselor can help you with that. Generally, the courses you need to take are like:
1 Year Bio
1 Year Physics
2 Years Chemistry (Inorganic and Organic)
1 Year Math (Preferably Calculus)

Plus the required Gen. Ed courses. However, if your counselor knows you are pre-med, they will make sure you fulfill this requirement even if you aren't a science major (however, being a science major will help you greatly on the MCAT exam).

There are several fields that you can major in. For example biology, chemistry, physics, etc. You should aim at the sciences, but occasionally some med schools take liberal arts students because they like to see a person that's not just all science. The med schools look for a good GPA and MCAT scores, extra curriculars, and a well rounded polite person! Hope I helped

Prerequisites for most medical schools are the following:

General Biology I and II
General Chemistry I and II
Organic Chemistry I and II
Physics I and II (Calculus-based, not algebra-based)

3/1 hours - Lecture/Lab

Some require a Calculus sequence or a specific number of hours in math at a certain level- check the school you are applying to.

Medical school admissions really like a diversity of courses, so I'd suggest taking literatures, foreign languages, etc.

As far as your four-year degree goes, it can be in anything.  The majority of med students have biology degrees, but it can be in English, history, math, liberal arts... anything at all-- just as long as you have those science courses listed above.