I'm 12 and I don't know how to ride a bike
Had you never learned how to ride a bike?
I never had anyone teach me how.
It might be embarrassing to point out that you have never learned something that most people know, but it's still good to learn new skills and to have new experiences.  Most of your friends or family will help you learn, because it feels good to help others.
Maybe this will be an interesting experience. Maybe it's a good thing I don't know how to ride a bike. It takes too much time to learn things, so I figure I'll do it at an older age. I rather spend my time on more important things.
Don't feel bad about asking for help. Everyone needs a little help now and then.