Most schools require letters of recommendation or evaluations filled out on-line by your evaluator. The evaluation option is new and it is important that you know what your school requires. They may require both.
When choosing people to write on your behalf, choose people who know you well. At least one letter ought to be a professor in your major. You will benefit from working at getting to know professors as you matriculate.
For more detailed information, see the pre-law advisor.