first you need to figure out what your budget is, then figure out who you need to hire. if it's a game or anything with pictures/animated characters / music / etc then you'll need at least one artist in addition to a programmer. then come up with all the requirements you can. draw pictures for every single screen, explicitly say what should happen when there's no signal, when user taps a button, etc. contractors can give you an estimate for how long they think it'll take. multiply that by their rate. if it's beyond your budget, then get more money or abandon it.
you can have them sign contracts that they won't steal your idea. but in all likelihood, nobody wants to steal your idea. everyone and their mother has a great idea for a mobile app, so there's really no shortage of ideas. not many people have thousands of dollars to invest into their app that will in all likely hood flop.