Just stay the way you are and eat healthy and you might eventually see results. Don't rush, next thing you get a huge gut - talking bout this isn't what i wanted.
Forget all these shakes, you want to gain weight, not be a bodybuilder right. So do it naturally it isn't hard if you want to pack on the pounds. Sorry to give you simple advice, but all you need to do is eat more, and more often, so you are increasing calories, which is the main factor. So we are looking at 5 meals a day, and a minimum of 500 calories each meal, so you would be getting 2500 minimum a day. For your weight, this would start putting on the pounds, then you would get up to 3000 calories, etc. If you start having proper meals and meal times it will happen quickly. So every 2-3 hours you should be chowing down. When you are eating 5 times a day, then start working out if you want muscle and not fat (which i assume you want). Don't be active and run around as much, forget treadmills and cardios until you are sustainably gaining weight enough for you to start excersise so in about a month or 2. Look at the calories and see how much you are taking in. Snack on nuts instead of the shit you are snacking on now, eat junk food and eat fruit do it all you can take in more than you think. Just get all the calories down you it's a win win situation as food is nice and the more you have the more you gain.
Start weight training at the gym. Don't eat unhealthy junk food. You'll probably put on weight naturally over the next 5 years. As long as you're healthy now, don't worry about being skinny.
Your metabolism will be super efficient at 18 years of age although it will slow down as you get older there are lots of helpful posts in this thread and i would echo some of them that urge you to increase your weight by healthy means rather than just eating lots of fast food it sounds like you would benefit from a little weight training as well coupled with a weight gainer such as nlarge2 or similar.
i think you should eat more food with carbohydrate it will help in gaining weight BUT still you have to balance your diet and exercise regularly
At the health food store they have a product called, "Weight Gain". Not sure what area your in, but they have this at Hi-Health. It is a powder you add to milk or water to make a shake, I don't like the chocolate one but the vanilla is really good. It is especially good if you blend it with a banana. In addition, if you smoke, you might want to quit. I think that speeds up the metabolism as well. Good luck! Edit: I just wanted to add one more thing. The important thing is to be healthy and try to accept yourself as you are. Have you ever noticed that women hardly ever just accept how they look? I tend to be too skinny and my best friend says she wants to be. She is more full figured, and I would rather look like that. If you have curly hair you want it straight...If your brunette, you want it blonde, etc. etc. It is probably the best just to try to be as healthy as you can be, I think.