I used to think my fast metabolism was a pain in the arse, i was just over 120 pound at just under 5' 11, i used to eat junk food all the time and never put on weight. My high metabolism just burned off those useless calories, it wasn't until changed my diet (in august last year) and ate healthier that i actually put on weight.
I'm 22 by the way, i was the same weight (120 odd pounds) since i was about 17. I jumped on the scales yesterday and was over 150, the weight isn't all lean muscle of course but i had so little body fat to start with i still look resonably lean.
So to get to my point (i do have one), your high metabolism will not stop you from putting on weight if you:
a) Weight train properly
b) Eat correctly
If you do this, you body will not burn off these useful calories, it will use them to create more muscle. Then when you do decide to eat some icecream or whatever, your high metabolism will just burn that crap off, great isn't it?
So if you aren't putting on weight at the moment you just aren't eating enough quality calories (plenty of protein, complex carbs etc), you might be eating 3000 calories a day which is a fair amount of food (might be enough for me to put on weight) but you might need 4000 or even 5000 a day to put on muscle. You WILL put on weight if you take in enough calories, it is just up to you to find out how many you need, so start eating and if you don't put on weight in a week, eat more etc.
This turned out to be really long sorry (i love talking about this stuff lol), i hope this helps and good luck in the future
Oh i almost forgot, if you haven't already, read DIESEL's bulking thread (it's the only sticky on this board), it will help alot.