This is the saddest thing I've read in a while twinklebabe. Nevermind being underweight, you must be suffering from serious malnutrition if all you ever eat is junkfood. You could stop smoking meth like Semi-Charmed recommended, but I'm sure you know that is the obvious solution to your problem.
If you're going to keep smoking meth, I recommend learning how to force yourself to eat when the thought of food is nauseating. Some really good, cheap ways to stop yourself from losing weight while smoking meth:
Drink the sugariest shit you can as much as possible. Never drink water, drink kool-aid or fruit juice or something. If you like iced tea, you can make homemade iced tea with the cheapest generic teabags boiled in a big pot on your stove. Melt a shitload of sugar into it and store it in your fridge. Drink a gallon of really sweet iced tea a day and you'll have a hard time not at least maintaining your weight. Just make sure to brush your teeth at least 3 times a day.
Start putting cheese all over everything. Especially vegetables. I find I can eat stuff like onions and green peppers, carrots, olives, tomatoes... all sorts of juicy vegetables, a lot easier than I can eat starchy or glutinous vegetables/grains when I'm not hungry or on anorexic drugs. A tomato and cheese sandwich with extra cheese is going to be a lot of calories, and isn't going to seem nearly as intimidating when you are anti-hungry.
Start putting olive oil all over everything.
Start eating more vegetables and fruits. Make fruit salads, garden salads, steamed veggies as much as possible. They are lower in calories than junkfood, but they don't fill you up for very long. It's easier to snack on fruits and veggies all day than potato chips and pretzels when you're on meth.
Avoid anything like potatoes, rice, bread, beans, greasy meat, and any deep fried foods. It's not that they're unhealthy for you, but at least in my experience, if I'm not hungry I'm more apt to take a few bites out of something like that and be unable to finish the meal. My stomach seems to feel full for a lot longer after a meal like that too.
Last but not least, stop wasting money on weight gainers, haha, they're a complete ripoff. The main priority in your diet ought to be, besides getting plenty of vitamins and minerals, eating calorie-dense foods. The more calories per serving and the less filling the food seems, the better.