If you have heard of app search optimisation, you already know how to grow your app organically. if not, here is a few readings.
Here is my blog post i wrote (very old) : Choosing a Mobile App Market Research Platform (not promoting my blog but its a no brainer article)
In short, this tools can help you to find what keywords you are ranking and what you should put in your description to rank organically. and other ideas.
For example, we use appfollow for a another project, it tells us how we are ranking.
You can signup for this getsecret and look for other deals as well.
The are tons of online article sharing APPSTORE optimisation tips