At 18 the body is still growing. Some people experience growth spurts after this age. It is my understanding that most people, even those who have stopped growing will experience a steady increase in natural Test production until their mid 20's. I would recommend staying away from any testosterone boosting products for a couple of years, at least until the body stabilizes a little more. I waited until I was 25 and worked out for 4 years before I got on the gear.
As for the tribulus, don't bother, it's pretty much useless. I have tried it myself with no discernable results, and everyone I know who has used it will attest to its uselessness.
I believe the best supplements for any natural person to take are creatine, glutamine, and lots of protein. Anything else is a waste of time and money. Except, of course, if you are trying to lose weight, in which case I have found an ECA stack can be very useful.
Hope this helps.