Honestly - try singing Baa Baa black sheep to one of your friends. It will give some idea if you can hold a simple tune which goes up and down the scales
Cats has made you an offer you can't refuse - audiences ARE extremely supportive to people who are just beginning.
Lot's of them would love to be able to sing but lack the ability or the confidence to do so , and they admire you for "giving it a go".
Maybe have a beer or a glass of your favourite tipple beforehand to relax and treat like a great adventure / journey into the unknown.
It is important to sing a song which you REALLY like ,in other words , don't try to sing a shanty ,if you're not a lover of shanties - pick a song that YOU like.
If all else fails , come and hear me sing , after which, you will almost certainly say to yourself , Jeez!!! I must be able to sing better than THAT.
Very best wishes and good luck for a positive result.