I need permission to play a song as intro or walk-in music.

This falls under a Public Performance License and is administered through ASCAP, BMI, and SESAC. These organizations collect royalties from songs played on the radio, TV, venues, and websites. The writer of each song affiliates with a specific PRO, and this will determine which organization to contact. To learn more, visit these websites:

www.ascap.com
www.bmi.com
www.sesac.com