To simply sing a song, or perform a song with instrumentation you do not need a license. Under Title 17 of the United States Copyright Law, section 110, churches can be exempt from copyright infringement for the public performance of a copyrighted work  so long as the work is of religous nature, and performed in the course of a religious worship service, however, if you perform a song and project lyrics, or make copies of lyrics you need a license for the public display of those lyrics. If you stream your worship service, record your worship service, etc.; that is where you need a license. CCLI (Church Copyright Licensing International) makes available licenses that will allow you to store, print, project or display lyrics, make custom arrangements, record your services, stream your services, etc. Check out all a CCLI license will allow you to do legally during your worship service.