Firstly, talk to a reputable freelancer/agency (depending on your budget) and give them a very brief overview of what you want. You will have to give them some detail as to what the project entails. Just simply saying 'sorry, it's a secret' will get you nowhere.
If they're interested get them to sign an NDA you've written and then give them a functional brief.
Also, I would seriously suggest getting away from offering equity as payment. Any decent developer worth his salt will run a mile. All it does it say 'I'm not confident enough in this idea to spend my own money, so I'll make you waste x months of your time developing it instead'.