That's a pretty loaded question, and people have written books about the subject so it's tough to interpret where you're starting at and what your current plan looks like.
My general mindset is that big muscles move big weights so focus on maximum strength (1-3 rep max). In contrast, high repetitions is more aligned with cardio (think about how a sprinter is built versus a marathon runner).
Speaking about your diet, it's easy to gain muscle...just eat as much as you can. HOWEVER, it's a lot tougher to gain muscle, gain as little fat as possible (you're going to gain some), and keep your blood sugar in check. If it were me, I'd focus on trying to gain 3-4 pounds a month (note: most of that will be water/food retention).
Be consistent in the weight room, consistent in the kitchen, and you will gain weight. Don't worry about supplements right now. In this context, anything that's legal is a waste of money.
Unfortunately, there are no secrets or shortcuts with gaining weight. You have to put in the time and sweat, and you can't just throw money at the problem and expect it to get solved.