Some cats will wear a collar for the first time with no fuss at all, but most of the cat will need some time to get used to a collar. The first time you put their collar on, they will usually find a way to strip it in moments.
So, as a process of getting used to an idea of something around their neck, put it on when they're indoors and you're there to keep an eye on them.
Try to distract their attention with some toys, treats or a noise; then reward them if they keep it on. If they are still refusing to cooperate, take it off. On your next attempt, try to persuade them to keep it on for a moment longer, and step-by-step they will get used to it.