Stick yer elbow out and lean against a brick wall - usually does the trick. As the guys have mentioned the sag should be between 20-30% - so for a 100mm fork about 20 - 30mm. I would guess that at 100kg you would need a extra firm spring. It probably came wth a medium or a firm. If the bike is new and came from a local shop you might be able to convince the shop to swap it for you.
The spring guide is hereWith the rebound knob, aim to start with it somewhere in the middle - see how many complete turns of the dial you can get from min to max, then half the number and turn it back. The faster the spring return (toward the hare), the quicker it is ready to take another hit, BUT if it rebounds to fast you will get a 'pogo' effect - this messes with your control of the bike.
Best way is to find a short length of trail and ride it repeatedly making ½, then 1/4 turn adjustments until you find something you like.