As Palikari says, at your age, you should need very little supplements, and especially not HGH.
If you're having trouble gaining, eat more and do less cardio. You don't have many pictures posted, so it's tough to see what you look like and how developed you are. That being said, at 18, you probably have a super high metabolism, so you are probably a really hard gainer (right now). Cut out any and all cardio for a few weeks, do some really heavy lifting, and eat tons.
By heavy lifting, I mean squats, deadlifts, pullups, and bench presses, as heavy as you can, with all of those, but ensuring that you maintain proper form. Proper form is much more important than the weight, but you need to work really hard to pack the muscle on.