I have to agree with
@socalfreak ,
@jockstrapattack . You are not an adolescent who can look at a plate of food and automatically gain 10 pounds of muscle and grow 2 inches. If you were still an adolescent, you could do half-arsed workouts and see incredible results. Sorry, but that ship has already sailed. You can still put on muscle, but it must be with EFFORT. Your very first response at the top of the thread to the first advice given was, "If I was being really ambitious..." , which to me indicates that you really aren't that ambitious.
Looking fit and being fit are two different things. You might get away with looking fit at age 19 without actually being fit, but as you get into your 20s, that will go away. Being fit is being athletic. Athletes train with seriousness. In order for you body to change at all, you have to work hard at it, eat right, and get proper rest. Once you get to the level you desire, you can cut back to maintenance. But you have to work out with intensity to get to ANY level of change.
I totally identify with being short and skinny as a rail, being totally afraid if a big guy even looks at me. Years of being bullied does that to a guy. I understand the defence mechanism of taking offence when someone questions whether or not I am serious about wanting to change my body. Probably the most crushing types of comments are the ones where guys or chicks will say something like, "Wait, you work out?" But you have to move past that and say that you are serious.
Send me a DM if you'd like, and I can give you advice on how to mentally prepare and adjust yourself for the sea-change that happens as your body changes. I admit that I'm still pretty thin, but I'm actually about 40 pounds heavier than I used to be.