Hey man. Well, as I see it, you've hit puberty a long time ago. It's normal to get aroused around a girl and puberty sex is nothing to be afraid of, or to avoid it, as long as you are protected and safe. So, I strongly advise you not to worry about your issue, because if it hadn't happened to you, then you should be worried. You're 18 man, young and adolescent. So, your first time will be a bit weird, but you will eventually get in shape and it will come on it's own, naturally. Don't worry about that.
Everything you are describing sounds normal. It sounds like you went through puberty and now are an adult who has a strong, normal sexual desire. In terms of the mechanics of how to do it, that’s something too involved to go into here; however, you should easily be able to find some helpful resources online. One piece of advice: use a good penis nutrient cream to keep your penis in fine health. One with vitamin A can help reduce penis odor.