One of your first stops should be Top of the Sierra Interpretive Center. Here, you'll be able to orient yourself and get some great ideas before you begin exploring.
Situated in Mammoth Lakes, this tourist information center is well worth a look. But you'll find a great deal more to check out in the area. Mammoth Mountain Ski Resort, Mammoth Mountain, and Lake Mary are within easy reach of Top of the Sierra Interpretive Center.