If the first gun needs to be a handgun, then a 22 revolver would be a good place to start imo. Or if they're at least familiar enough with handguns, then something like a Taurus TX22 would be a great starter. I'm not usually a taurus fan, but I do own a TX22. It's less money than comparable 22s, is basically identical in operation, disassembly and maintenence to a defense/combat pistol and it's a very reliable shooter. Also holds 16 rounds, so once you progress into training scenarios, its capacity simulates modern carryy/combat pistols too.
All the benefits of a 22 - low recoil, cheap and easy to teach, plus it's fairly representative of combat pistols.