Bob, there are a lot of levels of first aid kits. What's your intended use of the kit? Range/competition, blowout/EDC, trekking/hiking, general home kit, etc.?
As a place to start, every kit possessed by shooters (defensive or competition, it doesn't matter) should contain two things: A tourniquet and a roll of gauze (preferably the hemostatic type like QuikClot). Frankly, I prefer the z-folded gauze since it won't roll over the floor if you drop it and it feeds better.
Beyond those two things, you have to assess your intended use of the kit. That'll largely drive what you put into it (For instance: Our home kit contains burn treatment items like silver sulfadiazine and plain 4" rolled gauze).