How to increase body metabolism ?

Hiii guys. I'm 18 yrs old height 5'6 and weight 50 kgs.It has been 1 year since I've joined gym my diet is also good. But my weight is not increasing. I've heard that increasing body metabolism will surely help. So please guys give me simple ways on how I can increase my body metabolism. Also I've been suffering from allergic broncitis for almost 1 year. I think it has also made my body weak and slowed it metabolism. Please help me guys.