Setting the Work Area is very important. It allows you to define what portion of your composition is loaded for RAM previews as well as define which portion to render. You can set the Work Area by dragging handles in the Timeline Window, however there is a more precise method.
Position your Current Time Indicator at the beginning of the Work Area and press B (for Beginning).
Position your Current Time Indicator at the end of the Work Area and press N (for eNd).
Alternately, you can use your selected layers to define the Work Area. Simply press Cmd + Option + B (Ctrl + Alt + B) to set the Work Area to selection.