slow it down, don't start off with a bang showing off super speed or anything, the slower you are the longer you'll go, if you go super fast you'll blow within minutes but if you change it up a little, say start off slow and then speed it up then go back to slow it works a little better, you'll last longer too, if you feel yourself starting to blow just pull out and slap it against the inside of her leg.
With experience will come stamina. You are young and easily aroused. When you become more comfortable with having intercourse you will not tend to ejaculate as quickly. Masturbating before you plan on having intercourse will help slow things down too. Using a condom also helps with this problem.
well, this has happened to me, I have masterbated before going over to my gf's house before and when we tried to have sex my penis was too exhausted to "come out and play". If you get easily aroused then it might not be a problem...good luck