No need for a weight gainer supplement. Just aim to get your calories from whole, healthy foods. Work out how much calories you require to maintain your weight, and then add on an additional amount on top of this. 300kcals is usually a good starting place. If you end up putting on weight too quickly, then reducel calories... vice versa with if you're not putting enough on. 2lbs a month is optimal.
I recommend you start a programme like Strong Lifts or Starting Strength. These two programmes are excellent in developing strength and are highly recommended throughout these forums. These programmes incorporate all compound exercises and will also work your legs as well - you cannot ignore your legs!!
Can you also post up your diet please.