my new dewalt dcd771 drill won't start at all, battery's charged, what gives?

The trigger switch is locked out. The forward/reverse button is in the lock-off position.

1. Look at the button above the trigger. It has three positions: forward, lock-off (center), and reverse.
2. Slide the button out of the center position to either the forward or reverse side.
3. Now pull the trigger. The drill will run.

If that didn't work, the battery pack isn't fully seated. Press the battery release button, remove the pack, and reinsert it firmly until it clicks into place.

Heads up: Always place the forward/reverse button in the lock-off position before installing or removing bits, or storing the tool. This prevents accidental starts.