To add to what's been said, the key is consistency. You have to train consistently, add weight consistently, eat consistently, sleep consistently. What this means is: eat your target calories every day. Go to the gym every day that you're supposed to. Follow your workout every time. Add weight when it's appropriate to add weight. Get at least 8 hours sleep a night.
Not to mention: this will take a long ass time. We're talking years, not days or weeks or months. Some guys just don't gain weight easily, but that doesn't mean you can't do it - it just means you have to make the effort, consistently, for extended periods of time. Don't start now expecting to be beach buff in 8 weeks. You have to be in this for the long haul.
The good news is that maintaining the discipline will absolutely get easier. I started 8 months ago now and the routine is beginning to become habit - rather than effort - but I've still got a hell of a way to go.
The easiest thing you can do is:
Calculate your TDEE at https://tdeecalculator.net/. This is the rough amount of energy you expend in a day. Follow their guidelines for adding calories.
Sign up to Myfitnesspal (http://www.myfitnesspal.com/) and use it to track everything you eat. Bulking is easier than cutting but you still need to make sure you're doing it right.
If you need a workout, try Stronglifts 5x5 (http://stronglifts.com/5x5/). It sounds as though you're a beginner, so take beginner steps. Stronglifts is a program that guides you through barbell training, gets you accustomed to training on specific days, guides you through adding weight, etc. It's great for building strength and you *will* end up lifting heavy. After a few months of this you may decide you want to go for a more bodybuilding route, but remember - you're in this for the long haul. Build up your strength first and master the basics before you move onto more intense programs. EDIT: Also the app is amazing, so 100% use that.
Take pictures and date them. Same pose, same position in the mirror, same levels of clothing. Do this once a month and set a reminder on a calendar to check it in a year. THAT is how you'll see progress. Until then, just enjoy watching weight (both yours and the bar) go up.