Depending on how slowly you and your friend are willing to do this, I think starting oldschool with stuff like Doom or Wolfenstein 3D is best: Simple X-axis aiming with no Y-axis. Of course, this'd require to then go up the evolution ladder of FPS, play Duke Nukem 3D after it to introduce Y-axis aiming, then Unreal, and so and so forth.
If you're short on time/patience, then I'd reccomend either Half Life, set on the easiest setting, or, as odd as this may sound, Oblivion or Skyrim. Half Life has a very nice difficulty curve, and goes through most of the situations you're going to be having to deal with in most FPS. Both Oblivion and Skyrim have a very simple set of controls, are relatively easy, and, despite not having "traditional" shooting (bows and magic don't really work the same gun a would in an FPS), they're pretty good at teaching you to work under pressure in a 3D enviroment, which is really the biggest issue when learning FPS. Besides, whacking things with a stick is easier from a gameplay perspective, and can act as a nice stepping stone. Oblivion was actually my re-introduction to 3D games after a three or four year period of not playing (And before the hiatus, I'd only ever played those crappy Harry Potter tie-in games), and it worked like a charm. Skyrim seems more optimal, though, since you don't have to deal with the stat stuff nearly as much.
If you're short on time/patience, then I'd reccomend either Half Life, set on the easiest setting, or, as odd as this may sound, Oblivion or Skyrim. Half Life has a very nice difficulty curve, and goes through most of the situations you're going to be having to deal with in most FPS. Both Oblivion and Skyrim have a very simple set of controls, are relatively easy, and, despite not having "traditional" shooting (bows and magic don't really work the same gun a would in an FPS), they're pretty good at teaching you to work under pressure in a 3D enviroment, which is really the biggest issue when learning FPS. Besides, whacking things with a stick is easier from a gameplay perspective, and can act as a nice stepping stone. Oblivion was actually my re-introduction to 3D games after a three or four year period of not playing (And before the hiatus, I'd only ever played those crappy Harry Potter tie-in games), and it worked like a charm. Skyrim seems more optimal, though, since you don't have to deal with the stat stuff nearly as much.
