As was stated earlier, sing something you seem to FEEL, as that helps. Some of the older songs, Happy Wanderer, and such are popular many people would know them. Other popular songs if you feel comfortable with them would be Danny Boy, Roll the Old Chariot Along, Will the Circle Be Unbroken, and others in that vein.

Of course you can go into some sea shanties as those usually have a chorus which can be picked up readily. Shenandoah is one of those.

Other songs, not necessarily all that old, like the Green Hills of Tyrol or Flower of Scotland, people might remember from the radio.

It really depends on what songs you feel comfortable doing.