You need to copyright your music. ASCAP helps you collect royalties from performances of your copyrighted material.
ASCAP is free to join, lifetime membership and the membership can be passed along to your next of kin. You may only belong to either ASCAP or BMI but not both. If you are a member of BMI you must formally "resign" from that organization to join ASCAP.
To join ASCAP you must provide documentation to them of any single performance of your copyrighted material in any ASCAP sanctioned venue, radio station or other performance medium. At that point then they will send you a little plastic membership card, endless spam email about "I Create Music Expo" and once a quarter a statement showing you that you didn't earn any royalties.
You'd be better off going here :
www.copyright.gov and getting your stuff copyrighted instead, unless you plan on getting tons of airplay and other people covering your songs.
ASCAP member since 1996
AFM Local 689 since 1993