Hi
What you are going through sounds pretty scarey, but isn't that uncommon, I'm 40yo gay man, but way back when, I used to think about boys alot, dream about boys I knew, as strange as it may be, even straight guys do that too from time to time, but it really confused the hell out of me, for years, I have always known I was gay, but really didn't like the idea.
If you are using a gay chat site like Gaydar, then it can be a bit of a "hello, lets shag, cya" type experience, take your time, as hard as it may be, try not to jump on each other first chance you get, lots of guys, go looking for sex, when what they are really looking for are friends and relationships, if you fancy older guys, then try meeting a guy close to your age, so you can get to know each other or even become mates.
The feeling you have, could be any combination of things, the way you were brought up, the people you are friends with, all that nonsense that you hear about being gay as a kid growing up, try to think things through, calmly, it doesn't have to be that big a deal, you like who you like, no body can change that, its just the way it is, in time you might like younger guys.
Try no to stress about it too much, do what makes you happy, what you are comfortable with, I know at 18 sex is pretty high on the list of things to do in your spare time, but is that what you are really looking for, just sex.
I had better just put my "fatherly" hat on for a bit to, I knows its a pain, but be safe, use condoms, you can get them online, in plane packaging form places like,
http://www.freedoms-shop.com/ , in pub toilets, garages, supermarkets, all over the place, get some and used them.
Try not to let the whole thing become this huge issue, it really isn't, it's only a big deal, if you let it become a deal, how did a young lad I met put it, something like this.
Me "hello, you must be James, I'm stephens brother"
Him "hi, yeah, Steve and I work together"
Me "emma is really nice girls, she's looking for a boy friend"
Him "she is nice, but I'm gay, so she would be no good for me"
James is a really nice lad, for him being gay is like being blond, or short or tall, just not an issue, its just who he is.