ASCAP or BMI are going to be your two choices for a performance rights organization. The other in the United States is SESAC, however that org is essentially by invite only.
Before you register your music with a PRO, you'll want to make sure you've registered it with the US Copyright Office. You can fill out forms the old fashioned way and send them in, or you can file online using eCo. copyright.gov/eco/
You dont have to wait for your Coypright registration to be processed, but you should at least file it. If your music is used unlawfully and you would like to open a lawsuit for damages, you wont get anywhere without a copyright registration record.