Try this:

1. Loosen every single nipple until you can see just one thread on the spoke.
2. Starting at the valve hole, gradually (1/2 turn at a time) tighten each nipple by the same amount. It's going to take several times around the wheel to bring the spokes up to tension but don't try to speed the process by doing too much at once.
3. True the wheel by tightening and loosening opposing pairs of spokes by an equal amount.

Alternative, take the whole wheel to somebody who knows what he's doing.