How do i gain a little muscle for my girlfriend?

hi my name is justin. my legs are very strong as i bike and run everywhere. i could kick a guy's *** off. my age is 17 (as of june 24) my weight is 120, and everything above my waist is scrawny. my diet consists of 3-4 glasses of milk per day, i enjoy eating greens and i eat a lot of sushi (that's gotta be healthy, right?) so my bones are very stong, i have a lot of protein, and i have a powerful immune system. what home exercises can i do to gain some arm, chest, and back muscle for my girlfriend? i dont want to join a gym all i want is to use free time at home.