I really don't know anything about cutting grass!!?

General rule is to cut no more than 1/3 of the grass height, so you don't put it into shock. If your machine has a mulching blade (a regular blade, but with a higher angle, to lift the cut grass into the path of the blade for finer chopping -- you can get a mulcher blade for a few dollars if you want one) you can mulch dried leaves as you mow. I recommend bagging, though, if you don't remove leaves before mowing -- the process of decomposition extracts lots of nitrogen, so I don't think it's good for the grass.

Yes, you can cut the grass with leaves on it. Most lawnmowers take up the leaves as you cut the grass. A general rule for cutting your grass is, if the weather has been very hot or their has been little rainfall, or if you have been on holiday and come back to a field raise your blade so that your are not left with unsightly brown patches on your grass.