Hi A.,
As the other person mentioned, pre-k is for 4 year olds, but only lower income kids qualify. In the US, most kids do not start prublic school until 5 years old, which is kindergarten.
Many kids do go to preschool, part time or full time, either because their parents work and they need a daycare, or just because the parents want their younger kids in an educational, social envirnonment at a younger age part time (sometimes these part time preschools are called Mothers Day Out programs and are run in churches). Preschools are privately run, therefore you have to pay for them. There is no free public school for kids before pre-K.
There is a wide variety of daycare/preschool options out there from some that are strictly "daycare" to others that are more educationally based, so you might want to post to get recomendations for good preschools within your neighborhood.