I used to ride with a 17 year old guy in my group lessons when I first started. So it's not unusual, I think it's great that you're not afraid to and are trying to go after something that you're interested in.
As far as getting into it, just start looking around your area for stables that offer lessons, call 'em up and get started! Trust me they won't care that you're a guy because it's not as weird as you might think for a guy to want to do it. I mean look at all the cowboys! Not to mention show jumpers, eventers, even dressage riders (which is looked at by outsiders as the epitome of 'prissy, girlly froo froo riding' what those outsiders don't know is that dressage is the basis of all riding, not matter what you decide to do and in what kind of saddle :P)
Concerning which discipline you should go after? What ever takes your interest. If you're not sure and can't choose as far as english riding goes hunter jumpers is a good place to start to teach you how to ride "properly", from there a you can learn a little dressage and you can pretty much build on that to take it anywhere you want to go in the horse world. If you're leaning towards western you'd have to ask someone that rides western, I do not so I couldn't tell you were a good place to start there would be.
Depending on where you live may also kind of decide whether you're going to ride western or english for you. Like in my area, you'd be hard pressed to find a lesson barn that teaches western. So when you call around just ask what they have to offer, and maybe just warn them that you are in your late teens just in case they only have ancient lesson ponies that can't take the load of an older teenage guy.
But other than that, just have fun, do what you like and don't worry about what anyone is going to think about a guy riding horses...especially us horse people!