How to get a little bit fat because I'm too slim and I'm a dude
You can gain healthy weight by increasing the amount of calories that you take and eating nutrient based foods which are healthy. You should also do some exercise so as to help your muscles grow. Check out http://www.livestrong.com/article/540241-how-to-gain-weight-fast-for-men-eat-healthily/ for more tips on gain weight.
Eat foods like Ice cream right before going to bed late at night. Your metabolism is slower late at night and should help you get heavier.