Come on guys, all this can be easily answered in the owners manual. I'll tell you the answers if you promise to strictly follow the maintenance schedule in the manual, okay?

Sag - like hoover said - to decrease sag tighten the spring. This is called pre-load. Pre-load should be 4". Like DSride said - pre-load should be set so that 4" fo the shock travel is used from you sitting on the bike.

That screw on the bottom of the shock? That is the rebound adjustment. Turning it clockwise will slow the rebound. Turing it counterclockwise will speed up rebound. This will drastically effect the ride quality, and has nothing to do with sag, pre-load, or rider weight.

The screw on the side of the rezzy, under the battery? That is compression adjustment. it effects the speed at which the shock will compress. You'll want it looser for smooth rides, tighter for big air.