If you're only trimming the pubic hair, you should be fine. Clip it to a short and manageable length. The itching really comes into play if you shave down there; since it's your first time doing it, expect some irritation and redness and, though I wish you luck on it, possibly some ingrown hairs. Just like your face (assuming you shave it), you shave in the direction of hair growth to minimize irritation.
I shave the hair right on the base of my penis (the shaft) using soap and hot water in the shower; it feels decent, makes a clean shave, and minimizes pore clogging because the hot water opens up your pores down there. Just go sloooooooow with the razor. Other members around here reporting using "gentle" products like the Finishing Touch razors with mixed results. I can get away with using a Mach 3 that I've groomed my face with a couple of times; you know, get that last shave down there before I dispose of the cartridge. I suppose a disposable razor would work just as well, and it'd be cheaper; just remember, cheaper products might give cheaper results.
You know, if your girlfriend's such a stickler for you shaving your region, maybe she'd find it on the sexy side if you let her shave you? I dunno... just pitching that one out there!