The first thing is to work out how many hours a week you can spare, and whether you can manage to do regular hours or just odd days. Then you will know what kind of job you can go for.

If you're a medical student there may be someone in your department who could use an assistant of some kind - it's worth looking around anyway. If not, the usual next step is the college noticeboard, and after that the "part-time" section of the job ads in your local newspaper. It's also worth looking at the notices in big shops such as your nearest large supermarket, as these often advertise jobs in-store rather than through newspapers. Good luck.