The formal French phrase for the English word "please" is the phrase "s'il vous plait." When speaking to a person who is familiar, it is also an option to use the phrase "s'il te plait."
Most commonly, "s'il vous plait" is the phrase used to mean "please" when speaking to someone in French. In the case of speaking with someone very familiar, such as family or a dear friend, "s'il te plait" can be used as part of the conversation. The commonly used reply to invitations "R.S.V.P." is an abbreviation for "repondez s'il vous plait," meaning "please respond" or "respond if you please."