My penis is small and i am 19 years old. what should i do to grow some inches ?

Nothing: Assuming that your penis isn't small because it's concealed by excess fat or some deformity, you can't do anything. If there was something like that going on, i would hope your doctor would have discovered it a long time ago. You are who you are. I'm a full grown man and i'm five foot three and a half inches. Neither of us can do anything about it, so learn to be happy with who you are.