Buy a guitar. Play it to learn how to play guitar. Keep playing until you reach 5/10 level, which gives you the ability to "Play For Tips". When you have that ability, go to the park with the guitar in your inventory. When you are in the park, click the guitar in your inventory and select "Play For Tips". People watching you will become your friends if you keep playing. The higher your guitar skill, the faster they will become friends with you.