I'm probably just reitering what everyone said but...
You aren't going going to pack 50+ overnight - NOT happening. Get that thought out of your head right now - I'll wait............done? Good. Now you've learned patience. Don't get discouraged when you've been eating nonstop and at the end of the week you've only gained a pound.
But before we even get to what to do with your body / you have to be motivated. This is where I and many others have fallen before. You have to really want it in order to do it. Find someway to get motivated. Usually people are motivated by a couple of things: sex, anger, joy, money and power. You aren't getting money or power by wanting to get bigger so that's a no go.
Sex is self-explanatory: you want to be attractive to yourself and potential bed mates. Anger (this is my motivation tbh) gets you pumped: you're tired of being pushed around and teased because you aren't big and don't fit societies definition of attraction- they think you'll never be. You show em. You show em ALL. And finally, just basic joy in working out. You enjoy it do you do it. It's a hobby. Getting big is just a side benefit.
Second thing is to find a gym and go regularly. Not just once in a while - get a set schedule. Every other day. Everyday if you can...start off small by doing some machines just so you get used to it and then after some weeks start breaking your schedule and routine down into more complex workouts. Since your trying to increase mass, lift heavy with low (4-
But it will all go to waste if you don't EAT. You gotta eat a lot. Calculate your intake so you know how much you need to be a certain weight. That said don't eat garbage all the time. Them you'll just get skinny fat. You need full calories like peanut butter which is crazy filled that good protein - which you also need. Think one gram for every pound you have. You should also be sure to get a good meal in ASAP after your workout. I do use the protein powder that has 1200 cals but I'm terrible with it - need that twice a day. Don't rely on it though.