If I have a terrific idea for an App and I approach someone to develop it, how would that negotiation take place?

I have an idea for a healthcare service mobile application. I am acquainted with some people who have made apps and would approach them with this idea first. However, I do not want my idea (“intellectual property”) stolen, and I would like to be fairly compensated for the idea.

How does this usually work? How can I protect my idea while negotiating with potential developers?

Are the folks with the idea usually paid up front in a lump sum, or is there a way to get some royalties in the event that the App becomes profitable?