It is important to have a strategy when you begin your search for information. Start with the general and close in later on the specific. This will help you to understand what you are reading.
These are some common mistakes students make--avoid these and you will be able to write a better paper with less time in frustration:
1. Choosing a very specific topic, then starting looking at sources. (Sometimes your topic idea does not have a lot of information published about it. Look for a general topic first, then narrow down specifics after you have done some reading and learned what the interesting aspects are.)
2. Jumping into finding primary articles immediately. (Do secondary reading about your topic to narrow it down and get understanding. Jumping into primary articles first often means you are trying to pull together a paper from articles that don't relate to each other. This is very obvious to your professor).
3. Citing sources that are "too easy" and written for patients. (There are all kinds of web sites on medical topics that are not written at a scientific level--avoid these and stick to sources with scientific references).