I'm just 20 years old and my breasts are sagging. What can I do? I want them to be firmer.

There are many women in your age group with sagging breasts right from puberty. A breast lift can help with the sagging, but there is the trade-off of scars on the breast. Regardless, young women who undergo breast lift surgery are highly satisfied with the results. An implant may help give you some upper pole fullness. I would suggest you get some consults to see what makes the most sense for you. 

Thank you for your question.  Breast augmentations, lifts, and reductions can all be used individually, or combined, to enhance the appearance of a woman's breasts.  The best treatment recommendation would depend on an in-person examination or full series of photographs to identify your starting point, as well as a discussion to understand your desired end result.  See a board certified plastic surgeon in consultation