How can I advertise my app, without spending money? What would you do? What marketing method can I use?

1- Create a Facebook group.  Form a group for people with common interests related to app's niche so they can meet and socialize regularly.

2- Share content.  Use Twitter, Facebook, Google+ and LinkedIn profiles to promote the app several times in a week.

3- Be obsessive about resolving negative reviews. A happy customer is a brand ambassador for your app.

Market Method: Separate Yourself from Your Competition Find or create a reason for customers to do business with you instead of with someone else offering the same or similar products. For example, do you provide faster results, easier procedures, personal attention or a better guarantee?