Promotion is always the most important factor to reach customers. No matter how good your product is it doesn't matter if people don't know about that it exists so the best way to reach as many customers as possible is to make a free version of your app that will generate the most installs, and then you plan (before hand of course) a "pro" version that people can get once they upgrade. You should do this from inside your mobile app to make it as convenient as possible for your users/customers to purchase the upgrade. That of couse, as long as you :
1. Have a useful idea
2. The knowledge and tools neccessary to turn your ideas into a product
3. If point 2 is not possible find the right resources to help you/pay for progressing further
4. Plan to market your business/product.
5. Maintain and upgrade/support your customers.
Forget the idea to put out a payment app only. As long as people cannot get used to it they will never ever know what they miss.
Another factor is financial strengh and networking. Some claim social media is the best way to start, but I am not too sure about that.
The key is always to find the right way in. As they say 'the strongest chain always has its weakest link'.
An example, you won't go sell shoeshine at customers at Mcdonalds, but rather outside a store selling shoes. I hope you see what I mean.
Good luck is needed as well. Good people around you as well, to keep you motivated to carry on when some idiots think you're about to fail
EDIT: Learn to NEVER EVER have meetings with people who are not completely loyal to you and your visions. because those people are just wasting your time and causes frustration.
You will notice that alot especially with innovative ideas that easily causes jellousy.
There's always people who will turn their back on you while some show interest in what you do. Just be careful who you're talking to. A majority will ignore you and your great visions. I have studied just about every darn interview I could find with Steve Jobs, so if someone questions my points you will also question my mentor Steve Jobs. Nuf said.