I agree with avlehtine, you will definitely need springs, I am 190lbs and I had to go stiffer on both ends of my '00 CR250. You will find that the ride is MUCH more plush with the right spring.
As for settings, the small screw on the top of the shock is the Low speed compression, the larger, hex nut in the same place is the High speed adjuster, the rebound is located at the bottom of shock (down by linkage). Check out Jer's site (MX Tech) for tunig details, it's quite good, at least to get you started.
JTT
ps. You can turn the screw as much as you want, and should, you can always return it to original position.