How long do I have to wait before having sex while on the pill?

This will depend on what birth control pill you are taking, whether it's a combination pill or progestin-only pill.

A combination pill can be started at any time. If you start within five days after the start of your period, you are protected against pregnancy right away.
You will not need to use a backup method of birth control.
If you start at any other time during your menstrual cycle, you will be protected from pregnancy after seven days.

A progestin-only pill can be started at any time. You will need to wait for 48 hours (2 days) to be protected from pregnancy.

To protect yourself from pregnancy while waiting for the pill to take effect you must use another backup method of birth control — like a condom, female condom, diaphragm, or sponge if you have intercourse.