You can start trying immediately you stop taking the pill, but many sources advise waiting until you have had 3 to 6 menstrual cycles. It would be wise to consult your GP for best advice because he will know your history.
I came off the pill last December and I have only had two periods since, one in February and then one again at the beginning of July. Don't panic if you don't have a period for ages as my GP has told me this is perfectly normal, it can take up to a year. Try not to think about it too much. I thought I would be pregnant by now but I'm not even regular yet! It is, however, lots of fun trying!! Best of luck.