Now all important apps store Google Play or iTunes have hundred of thousants of apps, a user can find just a portion of that apps and for a new addition to the store it is hard to have some good ratings.
So what to do?
One think is to buy some ads. All bug apps buy apps on the Facebook, or on Admob or Google Adwords or on any other ad market you think of.
But one of the best and cheapest promotion is to order one or more reviews on different websites. and when you have this reviews and some video on Youtube you can share for yourself on the Google +, Facebook or Twitter.
You can check on Fiverr for some websites where you can order some reviews: