You have to do your research now if you have not done so already. Call the schools you prefer to ask when is their open day or talk. Go to the education kingdom here and research the schools by asking parents whose kids are in there. Don't believe whatever the headmaster tells you on open day, that is an advertisement only.
The kindergarten will give you forms in Sept of K3 (earlier for 直資/私小) and you have to make a choice then. Results of first round is around Dec if I remember correctly. Calculate how many points you have.
Those who are not allotted places in Dec have to go to the second stage: the lottery, all depends on luck. I think forms are given in Jan. There is a strategy in how to fill up the forms. Don't put schools in the same rank as the top three. One has to be lower in rank than the next, else you can drop to choice #4/5 quickly.