Well start researching your subject and choices now (if you haven't started already). I suggest you start writing the first draft of your PS before the summer, you can use TSR's PS Helper for feedback.
You'll apply sometime between mid-September and June. There are various deadlines (eg. for Oxbridge, medicine) and you're only guaranteed equal consideration if you apply before the stated date (should be in January).
For more info see UCAS (but keep in mind that these dates are for 2008-2009 applicants and there'll be different dates for when you apply).
I'm closing this thread now as I think you've received your answer. If you want to know more then please see the relevant wiki articles and useful resources. If unsure then feel free to make a new thread, providing it's a fairly sensible question. Thanks