For nausea, try not to do any suddent turns and what not and depending on the speed, use banking. If your ride goes fast over a non banked turn then it pulls some nasty lats. Try to limit the height of the drop. Don't make it Mr. Giganto unless you have a glorious layout in mind that will take advantage of the nice speed.
For Intensity, try not to make nuts.
Excitement, just try to make a nice layout and include inversions and a nice twisted layout. Also try to make your coasters compact rather than huge. You'll most likely get better ratings.
If you really need help, you can also download coasters to help and get a first hand look. Tycoonman and Shaggszgn make some awesome rides aswell as others that i dont want to list. Try to make your coasters sick and not sickening[;)]