How long after taking fertility pills can I start trying for a baby?

Immediately. Most pills are designed to be taken early in your menstrual cycle so you can have sex and try to become pregnant around the time of ovulation. This is in the middle of the cycle for most women.
Ask Your Doctor. Presumably, if you are taking fertility pills you have seen a fertility specialist. Why not ask this physician the question you asked us. He or she knows you better and can provide a more accurate answer based on your personal history and causes of your infertility.