Marketing is basicly getting a connection with the target audience of your product, in this case a mobil app/game. This can be done in many ways including the following:
- Your friends, family and acquaint. Talk to them if they have any interest in mobile games, else try get them to recommend your app to people THEY know like mobile games. This personal marketing can be achieved by mouth, phone, facebook, twitter, a personal blog.
- Try reaching out to a more "professional" network, which will include your fanbase, or other interested audience. A good strategy can be to have a facebook page for your game, a website, or a blog for updates on development or new content or other types of communication (see bigger other similar games that already top what they post on their networks).
- If you are completely new to the market, setting up a fanbase would be the place to start and this is mostly done by the two above steps in addition to contacting "bigger/more famous" people that can spread the word of your game. A few ideas to how this is done is to contact youtubers (serach for game reviews and "let's play"s to find people who does what you want), or streamers (does not really apply to mobile games), and have them do a review or playthrough of your game.
- Another little more costly effort can be add's, which you can purchase and place with help from google-ads or facebook-ads. It's then up to you to decide what kind of ad you want, where there are two major types of web adds. One is pay per click, and one is pay per view. They both have their advantages and disadvantages.
- Yet another strategy is to contact online magazines to write a review about your game, or a short article on who you/your company if any, are and what you do.
This are some good starting points, some more difficult than others.
What you must begin with is learn to "speak for your product", which is basicly the content you want to display to other people, to have them interested in your product. A good place to start is by searching for other adds, and reviews, or basicly any material related to a well selling game and see how they give their game appeal to others.
Here is a good post from reddit on promoting a game. It is relevant here.
Here is a guide on how to contact the press, written by and for indie developers.