I don't understand how people can put their pets to sleep.. especially if the pet is just old and isn't as active as it used to be. 12 years isn't even that old for a poodle, they can live up to 20 years if you get lucky.
So don't prepare for anything, take it to a vet, get it checked out. I'd never put my dog to sleep unless he will die in an hour and is in a lot of pain and there is nothing to be done (incurable disease or something)