You've received some brilliant Answers and advice.
My approach is different: Confidentially (or with a formal non-disclosure agreement, your choice), I will listen to your app idea pitch and then do my best to convince you to NOT proceed. My introductory service is Risk Management -- That means I start out as a professional "wet-blanket", and I believe that every idea and dream requires a thoroughly wrapped soaking. I take no pleasure in destroying dreams; to the contrary, my job is to save you from a hopeless endeavor, or to harden your resolve to prove me wrong. I love being proven wrong.
If you or your dream can't survive my cross-examination and possible suggested pivots, then that's a sign you will fail, even with substantial personal commitment and investment. You could be among the 92% of *funded* technology startups that fail within 3 years. It is estimated that far fewer than 5% of active Apps earn more than $1000 USD per month in revenue. Other statistics may be more or less pessimistic; one example states that the average *total* revenue for an app is well under $10,000 (i.e., after sales of the app have petered out).
A brilliant idea is a good start -- if it truly is "brilliant" and leads to a sustainable ongoing revenue stream. Execution consists of far more than getting the app "developed". App development cost is only a fraction of what you will need to spend to build a potentially profitable venture. The key to success, as low as that chance may be, comes from partnerships, marketing (very limited value from "social media", by the way), and a *lot* of luck and perseverance.
An app developer that charges a set fee to complete the app is taking no risks. Most such developers earn below the poverty line. The others will expose you to tremendous up-front costs while just trivially increasing your chance of success. An app developer that will work for an equity percentage in your venture, is only worth considering if they have no or very few other such arrangements to fall back on. In my opinion, these latter type of developers are the ones that are the best choice. They are dedicated to your idea and will be partners in your success or failure. They will invest in the iterative, incremental process required to prove the viability of your app to investors. Indeed, I could be *that* developer for your idea -- if and only if I share your dream.
Give me a call and I will let you know if you have a chance at success. And then, IF you are really cut out to make your dream come true, you will firmly resolve to overcome or ignore the pessimistic conclusions I usually reach.