When you get out of college, or even sooner if you look for an internship or a summer job, you might be asked by employers or graduate schools to provide a letter of recommendation. Such letters are most often written by a professor at your school, in order to attest to your interest, knowledge and skills.
The upshot: You want a letter from somebody who is actually familiar with you, and who can write about the kinds of things that your evaluators will be looking for. But how do you know whether someone knows you?
If you have a specific question about how to move forward, you should approach an advisor, a faculty member, or someone in your advising network. People are eager to help.
To get a good letter of recommendation, you'll want to be sure that your letter-writer has a chance to get to know you. One of the easiest ways to accomplish this is to visit your professors during their office hours and ask questions.
Another point to keep in mind is that it's best to ask for letters earlier than you might need them. If you know that you're going to want a letter from a certain professor, it's better to ask right after taking their class than a few years down the line (since memories fade over time).