It may not be sexy, and might have a very low cool factor around your friends, but a good S&W K-frame .38spl revolver would be your best bet to start out with. Manual of Arms is easy to learn, recoil is mild, .38spl is a respectable cartridge, and the K-frame is easy to conceal. Most if not all K-frame revolvers are extreamly accurate. These three rules apply regardless of the gun used.
1. Shot placement is whats its all about.
2. Penetration is whats needed. If the round doesn't get to the vital areas your not going to stop the threat.
3. Hollowpoints....... They are not the "majic pill", sometimes they work, sometimes they don't. Always refer to rule #1, an #2.
I was gonna suggest a good ol' model 10 but I figured with him being 24 years old...