Jesus this thread....
Alright OP, ex/fit/ here, cuz I left once I got what I needed. They are chronic faggots.
You need to find a bunch of quick,cheap, and calorie/nutrient dense food.
Any food eaten enough will help you gain weight.
You simply need to eat more calories than you burn. Being a 6'1" 18 year old skinnyfag means your metabolism is fucking high.
It also means your cursed with eating a fucking huge amount each day. Although in your situation there is one small benefit: you can eat however much you want, however you want.
The best option would be a controlled intake of protein, clean carbs, and controlled fat intake. It sounds like your a poorfag too, which is unfortunate, but workable. This means that you will need to get a little fat before you get muscle.
The "dirty bulk" instead of a clean bulk still isn't the worst idea in your place though.
The best options for a poorfag depends on exactly how poorfag you are, or how far you're willing to go mentally/financially.
If you can handle it, the best answer to your problem is buying rice, oatmeal, peanut butter, eggs, and cottage cheese.
All of these things can be bought in bulk at sams/Costco for almost dirt. Any dairy product would also be useful, depends on what you can afford.
On top of that, if you're not a third worlder, eat anything else you can get your hands on all day every day.
Now, you need to fucking eat. If you can do basic math, attempt to get 3500+ calories a day. Also drink a good bit of water, about a gallon a day, regardless of food intake.
If your not shitbroke, throw in other shit to mix it up, can be anything. Chicken, beans, bread, OATZ, fruit, veggies for clean eating, but pizza, ice cream, etc is good all day long as well at your weight. (Cuz protip I cant imagine how much of a spooky fucking skeleton you are)
Honestly, all anyone can tell you at this point is just eat your ass into a new size/1