Need Help! My 15 week old German Shepherd puppy keeps biting me!?

Hello, i want to know peoples tips and ways to get my puppy to stop biting me so aggressively. It has come down to that he now doesn't bite my mom, dad, or my older brother... he only bites myself, 15, and my little sister 6... whenever he's in the backyard and i step out he'll just start to bite my legs and its getting annoying and he's drawn blood many times... any tips.