Ask the classmate what specifically he needs help with and find the resources to help. If, for instance, he needs to learn how to conjugate verbs in Spanish, then you may want to review the conjugation charts in your text book or create one on your own to help your classmate.
Test the classmate's knowledge on what you have covered by having that person perform a mini-test. Create one yourself from the material that you have studied or you can use one from a textbook or other study materials. Mini-tests do not have to be elaborate. They can be as simple as flashcards made from index cards.
Collaborate creatively if you are assisting a classmate with a project that both of you are to be graded on. If there is a portion of the project that you are better at than your classmate, then do that part. For instance, if your project involves drawing pictures or sketching objects of the solar system onto poster board, and this is your talent, then volunteer to do the drawing part of the project. Your classmate may be able to write the captions or research information on the planets. Make sure that you complement each other's weaknesses and strengths, which can help make your time on the project more enjoyable and productive.
Refer your classmate to materials that were helpful to you if he or she asks for help on material you don't remember. In other words, if your classmate is an underclassman and asks for assistance on material in a class which you, now an upperclassman, took a year or so ago, let him borrow your old books and the tutorials that you used. You might also let him see some of your old notes, depending on your school's policy.