How to get her
I have a headache just thinking this through. If you're a straight girl or a gay boy, no, you haven't suddenly reversed your sexual orientation, throw cold water at Ranma and you'll see what I mean, then go here. If you're either a lesbian or a gay boy, arrange for Ranma to get hit on the head with something really hard. Then to avoid unwanted complications, make sure she's never withing ten feet of hot water.

Competition
My headache just graduated to migraine. Okay. You have TONS of competition, among the guys, the one that might give you the most trouble is Kuno, if only because he's nuts and will never accept that the pig-tailed girl is not madly in love with him, and he'll probably beat you up if you persist in chasing her. The girls are another matter. Most of them are only interested in boy-Ranma, so if you find a way to make the sex-change permanent, I'd say that's that.

She likes you if...
Don't trust her, she's a vixen who's probably using you to get free ice cream. Unless you did the hitting on the head thing, then she's a sweet, innocent thing.