Regarding being thin and wanting to put some strength on, there's a very well written answer over here I'd recommend checking out.
I usually skip meals or eat little. I drink a lot of juice (Apple
Juice) which I know has a lot of sugar.
Juice is pretty terrible to be honest: it's like all the bad parts of fruit with none of the good parts. Your body is supposed to work in breaking down the fibers and unlocking the sugars. By doing that for you, the sugar absorption rate skyrockets and it's on par with drinking a sugary soda. Check out some stats on it:
Will I lose muscle or fat when
I am "fasting"? I don't mind losing fat, but losing muscle (which I
don't have much) concerns me.
You will lose muscle under three conditions. If you combine them, you'll lose even more.
You're not requiring a lot of strength. Your body will only maintain the muscle strength it deems necessary. If you are not regularly subjecting it to high loads, it will save itself the trouble of keeping "unneeded" muscle mass.
You have less testosterone and human growth hormone, amongst other metabolic issues. This happens as you age, if you don't sleep enough, and if you have an endocrine disorder.
You are in a caloric deficit. Your body primarily wants to keep you alive, and if it needs calories it will get them from you via cannibalizing your body. This is really quite linked to the strength requirements. If you are not loading your body with strength requirements, muscle becomes a nice calorie rich target.