Start searching: it's probably best to, believe it or not, start looking at schools at least a year prior to attending. Look in the area, ask neighbors, get in touch with schools. Check back in with schools that interest you - some often have waiting lists. Try attending some preschool fairs to get recommendations and a lay of the land.
Schedule a tour for those on your short list. Schools will have open houses or you can schedule a tour through they office. At your visit, here are some good questions to ask:
Do you have a guidebook?
What are the qualifications and credentials of the program director and teachers?
What ways does the school keep parents informed about their children's progress? What's the typical progression over two years?
How many students are there for each teacher? (Tip: Rule of thumb, for 3 year olds, 12 students for every teacher is often recommended by most state authorities and organizations)
Understand how often and how long you plan for your child to attend. Two years? Half-day? Full time? Are there special needs? If half-day, will you need additional childcare (Tip: Montessori schools often have longer days)?
There are five domains of school readiness according to the National School Readiness Initiative. As you start to narrow down a list, it's good to assess if the program supports each:
Cognition and General Knowledge
Social and Emotional Maturity
Approaches to Learning
Does your child need potty training? Many schools require that children be potty trained, but not all. Figure out a plan depending on your child.