Don't apologise! It's an annoying habit of many amateurs and even some professionals. Don't say, for instance, that you've got a bit of a rough throat tonight; that you've only just learned this one and may get it wrong; that you usually sing this one with another person or instrument and it may not sound right; that your guitar strings keep slipping - or if unaccompanied, that you aren't sure if you're pitching it right.

Most of these problems can be anticipated and dealt with - throat sweets, tuners, pitch pipes, learning your material, etc. Apologising in advance just unnerves your audience and makes them look out for faults. I have heard very famous and successful singers repeat a verse, change the pitch after a verse, etc, but if they do it with bravado they get away with it.

Good luck!
Marje