The collar on the shock and the nuts on the top of the forks adjust spring preload (which actually is not preload, but thats what its called). This affects the ride height, but indirectly, so you shouldnt use that for that perpose. The idea is to set your static sag, in other words, how far the suspension compresses when you sit on the bike. The suspension should compress about 1.25" with you on the bike. This way, it has plenty more to compress when it hits a bump, but if you come over a pothole, the suspension can extend and reach the ground. However with your weight, I'm guessing you will not be able to get the right sag with the stock spring (you might, but I doubt it) and will likely have to get new ones. You can get them from race tek for a couple of hundred dollars.
The other way to adjust ride height is by using linkages on the rear shock and sliding the forks further down in the tripple trees. The important thing is to adjust both the front and rear by the same ammount. If you dont, you will change your rake and trail, which could make the bike less manuvarable, or dangerously unstable.