Find a professor you actually like. Yes this is important, you are working for them so find out if you can tolerate them. Next do research on their research. Find their grant proposal if applicable and read it fully. If they spent time asking for money, find out what for and why. If it looks like you would enjoy what they are doing, shoot them an email if you don't know who they are. Ask if he/she is looking for a serious research assistant. If you know them, drop by their office and just talk to them. Rejection is part of the process, if they say no then move on.