There are two main positions on the bike: in the saddle and out of the saddle. Riding in the saddle should be your default setting, but for short bursts and accelerations, ride out of the saddle.
With the two basic positions covered, you can choose your hand position based on your speed and desired level of comfort or aerodynamics. There are 3 basic hand positions on road bike bars. The fastest and most aero positions are riding on the drops or on the hoods with a flat back.