I can't for the life of me figure out how to pick what kind of ammo I use. I keep buying special ammo, but only use standard shot...

Aquire the different ammo types for the weapon you wanna use (Standard,AP,or HP) then press up on the Dpad while the weapon is in hand.

It looks like Numeric 2, HotKeys, is hard wired to Ammo Selection on the PC.

This, pressing 2 will cycle through your ammo available for that weapon. You can also go to the ammo window in inventory and select the one you want.

Yea there is hotkeys on PC, up D-Pad on consoles, or can just go into the ammo section of items in the pip-boy in any version of the game, anything your current weapon can use is bright, not faded out.