A.S.
I think it depends on your siuation. I know that a lot of stay at home moms start their children in preschool when they are 3 and they go for 2 years. My children have gone to day care and have a lot of socialization. They are both bright kids and so preschool, for me, is to introduce them to routines that they will need in school - sitting and listening, waiting for turns, and more structured activities then they get at home or daycare.
If your child isn't around children much, 2 years would probably be better for social behaviors and skills. Same thing with the academic skills. If your child needs more help with that, might be helpful.
Typically, Preschool begins with the school year and registration happens in early spring for the following fall.
In terms of finding one, that can be tricky. Our local preschools put ads in our local newspapers. Once you find a couple. Visit each one for a morning and ask for references and CALL the people! Parents are honest and you can tell a lot from what the parents say - even when they don't realize they are!
Good luck to you.