I can't remember what the exact percent is, but I believe the fleet is composed of at least 50% helicopters. The rest of the fleet is a variety of aircraft that include jets (not only F/A-18s).
As statesmen said, check out the ROD (regulations for officer development). However, this is an extremely thick document with a lot of administrative things that either don't apply to you or can contradict each other.
What specific things in the fine print are you looking for? I recommend contacting the NROTC school your interested in and speaking to the designated recruiting officer. They are not out to trick you and will give you the low down on what commissioning consists of.
If your child is going the OCS route, I don't know as much about that except that they call the Eisenhower a "relax jacket."--an attempt at humor.