Well, there's that pesky background check, so preschool teacher is probably not a good career for an ex-convict trying to turn his life around. If your record is clean, your best bet is to get educated. Helping out or volunteering is a great way to get started, but to be a career preschool teacher, you should go to college. Fortunately, there are many great schools with excellent childhood development programs. The job growth is expected to be much higher than the norm (25% vs. 14%) and schools know it. Take advantage of a surprisingly large number of institutions offering degrees in childhood development.
The need for preschool educators won't go away, but the U.S. is undergoing a shrinkage in newborn rates. Meet that competition while equipped with a higher education than other candidates. Otherwise, you can get started as a floater, a sub, or a "childcare worker" and earn about $20k a year.
The odds are also in your favor if you work in a daycare setting rather than a public or private school.
A second language or a focus in special education can also be a huge résumé booster.