Apart from all of the above good advice, one other suggestion made by singers I have encountered, is to choose songs YOU like, not necessarily ones that people have told you "you really should sing   xyz". It is much easier to make make a convincing and appealing rendition of a song that grabs you yourself, and tell people why you think it's relevant to you.
That doesn't mean you don't need to work at it: as Jim has said, there's still making sure you understand the song, phrase it well, put your heart into it, etc.
Note down any that you hear in sessions that you think you might like to do: fortunately these days we are blessed with many online resources, as indicated above (and let's not forget Mudcat itself - my first ever online resource for song lyrics.)