WHEY PROTEIN, MULTI-VITAMINS, FISH-OIL
Those are definitely the 3 most important supplements for training. Make sure you are also eating A LOT and THE CORRECT WAY. If you are still having trouble gaining weight/strength(which is basically impossible), then you can try some Creatine and/or NO's. I'm personally not a big fan of supps and think they should only be used to SUPPLEMENT your diet/workout regime, not make it.
What a stud
The power of Aleks, his small baby and pimp hardcore dogs has granted you useful insight, my friend.