Learn the ASL alphabet. In sign language, words that don't have a dedicated symbol are spelled out using the alphabet, so not only is this an important fundamental skill, you can use it to fill in the blanks while you learn other words. The alphabet also includes few complicated movements, making it a good basic first step in learning ASL. Print out a diagram of the alphabet and keep it on hand to practice and memorize, then start spelling out words, names and simple sentences to gain mastery and speed.
Find a friend or group of friends to learn with. When learning any language, the best practice is to have actual conversations, so set up regular practice. Also, take advantage of the silent nature of sign language by having short conversations in situations where you're supposed to be quiet; you may be able to convince your friends to join in the fun in this way.
Use video tutorials and a mirror. The signs for many words involve not just hand shapes, but hand and arm movements. Reproducing these accurately using only diagrams is difficult, so watch videos and imitate the gestures. Watch yourself in a mirror and pay attention not only to the gestures themselves, but to the rhythm and speed at which they're performed.
Attend events with ASL interpretation. School-sponsored events as well as many professional theater productions regularly hire a sign language interpreter.
Start doing your own sign language interpretations. As you gain mastery, expand your skills and vocabulary by interpreting television shows or song lyrics on the radio as you listen to them. Make a note of any words you don't know and would look up signs for.