If you don't eat enough your not gonna get big
The number one tip is to eat more!!! I know this is pretty obvious, however if your skinny you may not eat a regular intervals or you need to suppliment your calories through protien shakes and snake bars.
One of the things I try to do is eat 6 meals per day all around 400 - 500 calories each, this way your blood sugar is stablilzed and you burn fat.
You can subscribe to his Mikes channel HERE I love doing his workouts at home as its straight forward and time saving workouts.