Most undergraduate applications require two or three letters of recommendation. These letters should come from former or current teachers, employers, or supervisors who are familiar with your work and performance.
Academic letter of recommendation
For academic applications, letters from teachers are generally preferable to letters from employers. Admission officers are looking to supplement their knowledge of your academic performance and aptitude — gleaned from your transcript and standardized scores — with concrete evidence that you are a dedicated and enthusiastic learner that will enhance the college culture.
Most schools nowadays recognize the value of a dynamic, diverse student body and are thus eager to fill their spots with candidates who have been actively engaged in both academic and extracurricular activities. Your college letter of recommendation should reflect not only your participation and performance in the classroom, but also your initiative (for instance, through research projects undertaken, through leadership in group activities, and through active contribution to classroom discussions).
Seeking letters of recommendation
When you seek a letter of recommendation, remember that you are asking for a favor. Make the process as convenient as possible for your letter writer by providing context, direction, and even stamps!
Most importantly, give your contact ample time with which to write it. A hastily written letter won't have the same impact as one that's reasoned and well thought out.
Other elements of the admissions process
Don't forget to work on the other aspects of the college admission process as well, such as sending transcripts, taking standardized tests, and scheduling a college interview (sometimes called an admission interview) at the schools you're interested in.