Formal:
* Un instant, s'il vous plaît. (Literally: "A moment, please.") This is the most formal and polite way to say it.
Informal:
* Attendez un peu, s'il vous plaît. (Literally: "Wait a bit, please.")
* Une minute, s'il vous plaît. (Literally: "A minute, please.")
* Tenez bon, s'il vous plaît. (Literally: "Hold on, please.") This is a more direct way to say it.
* Patientez un peu, s'il vous plaît. (Literally: "Be patient a bit, please.")
On the phone:
* Ne quittez pas. (Literally: "Don't hang up.")
* Restez en ligne. (Literally: "Stay on the line.")
* Veuillez patienter. (Literally: "Please be patient.")
You can also use informal expressions like "Un sec" (a second), or "J'arrive" (I'm coming), but these are usually used in situations where you're only going to be away for a short time.
The best way to say "hold on please" in French depends on the situation and your relationship with the person you're speaking to.