I have an anxious child for such things.
I have two girls, one of whom got anxious before starting, but her wanting to do it would conquer her fears. She'd take a term before she was used to it generally, but would be nervous and unsure each time. Second term would go in fine.
My second girl loves new things. Will be the first to dash in and always was, will come out at the end of the first session with a best friend etc.
Now I have ds. Oh dear, it's a totally different game. Even though he's 9yo he still struggles.
He will always panic before something new even if he knows he will love it and was totally on board when I signed him up. He'll wobble about it for at least 6 months (yesterday he said "how can you expect me to do things in front of strangers-I've only known them 6 months!")
So he'll panic before going-but then comes out happy and having loved it, but the next week he'll panic again. It's very frustrating. And he's worse after a break even a week.
He was ill last year for half a term and basically dropped everything he loved doing that he'd got used to. I'm now having to build him back into doing things as the more he stays at home the more he panics about doing anything. It's very difficult.
What I do is bribery for him. I do only do it when I think he'll like it. So at present he loves McD's. So I might say "If we go to X then we could go to McD's afterwards."
Let him think about it and then he'll either come to me and say he still doesn't want to go, (and then I'll consider whether to do further bribery, tell him sorry but he is going (very rarely and only for specific reasons-eg he'll be letting others down) or decide it isn't worth it and pull him out) or he will say (very sulkily) that he supposes he will do it, have a few tears on the way into the car, but then will be fine once he's there.
I wouldn't ever force him to do something he doesn't seem to enjoy when he comes out.
There are a couple of things he loves the idea of joining, and he talks about joining quite a bit. When I suggest I ask about a trial session, he think he might like to do it... next term... possibly next year... when he's really old (like 12yo). I am choosing not to sign him up for them at present, even though I know he would love them because I really don't want the fight at present to get him to the trial sessions. It's hard.