A hammock won't work at all in the desert. In fact, I would avoid the desert completely this time of year, particularly as a beginner. Daytime temperatures at Joshua Tree will be over 100 degrees in the shade. Desert hiking is best from late fall to late winter.

As you work your way north you have literally hundreds of hikes in the Sierra to choose from. Personally, I would stay away from the crowds in the national parks and focus on lesser-known wilderness areas like Golden Trout, Dinkey Lakes and Emmigrant.