It depends whether you need it to be a Christian song ( I am assuming that is what church type you go to, sorry if not) or just a normal but non-suggestive song.
For a Christian Song, you could always go to the Book of the Bibles "Psalms", make up a tune, and sing that, or, every church camp I have even gone to sings alot. Listen to what the camp counselors or singing. If you like one of those songs, work on memorizing it while at camp.
If it can just be a regular song, sing "We Are Gonna Be Friends" by the White Stripes. It is really easy to memorize and sing. Even better if you have a guitar you can play. Another good way to go would be taking a well known song like something by the Beatles for everyone to sing along to, to ease off the pressure.
Of course, whenever I have to do a talent show, I just copy something off of Brian Reagan or Demitri Martin or Seinfeld or something because comedy routines are so much easier, and usually just a bit more entertaining.
