Short answer: eat. A lot. Not to the point that you constantly feel full and sluggish, but learn eat even when you're not that hungry. Chicken breast, lean beef, penut butter, wheat bread, milk, and a lot of other foods are great for this.
And start lifting heavy occasionally. Whatever you can safely lift for no more than 6-8 reps, maybe less, should be good. And start squating and deadlifting if you aren't already. But like others have said; don't just jump into heavy lifting without a little help and/or guidance.
It worked for me and I'm currently a lean 5'6 and 170. But there are a ton of other methods that may be more effective and/or easier on your body though, so just keep experimenting.