I wanted a little info about how USAAF/USMC/USN designated their squadrons on their planes. I know there were tons of tail painting and such, but was there a basic way that all squadrons were required to use. I also wanted to know what the letters on the side of the P-51s meant.

Unlike the USAAF, the USN and USMC didn't go for much squadron identification once the war started.   Initially, and this was a pre-war carry over, Bombing 6 (Dauntless, CV-6 USS Enterprise) would have a 22-B-6 on the side of the plane, indicating plane No.22 of (B)ombing squadron 6 where the 6 refered to the carrier.

Later, as squadrons were rotated, and the large number of squadrons were created, this was changed.

Squadron markings, such as VF-17's skull and crossbones flag on its Corsairs, or other small squadron patch, could identify a particular squadron.

Later in the war, geometric designs on the tail marked a carrier wing (such as a white triangle marking the USS Enterprise)

Side note on the RAF... one squadron kept its squadron markings (blue-red-blue bars arrowhead) on its otherwise camouflaged aircraft into, I believe 1941 (Middle East)