Q: I'm really small and have very small boobs. What can I do to get them bigger in a healthy way and still stay underweight?

A: Sorry, that's not something I'll be able to help you with. You could do chest exercises like pushups or the bench press, but the increase in your chest size is very unlikely to be noticeable unless you gain significant amount of weight.

The only sure way to increase the size naturally is to put on some fat which may not be something you're interested in. A traditional 'bulk' program (where you'd try to primarily gain muscle without gaining too much fat) would also be inappropriate for you since that muscle won't do much for your boob size and may make your body excessively muscular.

So, there's no good advice I can offer unfortunately. About the only thing you can do naturally is try to gain some weight and see if you like your appearence better then.