If you're just working on an MVP you can use a freelancer sight like Upwork, but you have to be careful in your hiring.
1) Post a job stating everyone that applies should give you their github / bitbucket username and Trello username.
2) For the top 5 most promising people to apply, give them a simple task which should take about 30 - 60 min for a decent developer to do (e.g. implement this screen with these one or two actions).
3) Pick the developer that actually delivers exactly what you asked for the fastest, and in the most professional manner (not asking a million questions, etc.).

To assign and track this and subsequent tasks use a Trello board with three columns:
To Do
Doing
Done

Ask the developer(s) to update the Trello cards in real time so you can keep track of what they're working on and for how long.

If you'd like any more specific advice in relation to your specific app idea let me know,

best,

Lee