If you exercise, how long until you start seeing results?

Hey don't worry about gaining weight. For guys it is a bigger issue. Eat healthy, do not miss your meals. You definitely do not want to change your body type. And usually, after the first two weeks working out you will feel much better, concentrate on how do you feel than how do you look. The results depend on how intense your work outs are, and how much rest you are getting. Results can be seen from 1week to 4weeks. But the main thing is for you to keep doing it, live a healthy life style. In order to keep doing the work outs you need to have confidence, self esteem, and feel good after each work out.
I've been training for a while, and now I train people also. Message me if you want some pointers.
read this also: link

3-6 months, but make SURE you eat right, lots of protein

Sometimes you'll see results in 6 weeks, but you must remember that most the changes going on are inside you, and what you see on the outside may not reflect the changes inside you. You should exercise every other day, so one day exercising, next day resting, and so on.

You also don't say how old you are, when I was 17 I always thought I was too skinny, ate 6 meals a day for about 18 months and didn't put on a single pound, as soon as I hit 19 I started to bulk out no matter what I ate so don't try too hard because you'll just stress yourself out, some people are naturally skinny, nothing to be ashamed of, probally better for you in the long run.

Also, if your working out and not eating to support it you'll lose weight. Just eat healthily and exercise reguallary, nothing too intense, and that's the best you can do.

I've been reading seventeen for awhile and each time I read their fitness.they usually say that results are about 3 weeks but it's really small though.

Good luck on gaining weight for someone that's naturally skinny. it's hard. haha