Start by seeing what you can do for free - a good thing to begin with is emailing or submitting your game to indie review sites, most of them have a 'Contact Us' section or a webform where you can give them details of your game. If they review or feature your game then that's a whole bunch of people that will see your work and may go and play it. Also sign up with sites that will give you additional visibility online - for example we use http://www.indiegamelaunchpad.io/ and they tweet a link for a different game out to their 12k Twitter followers every hour.
In terms of paying for advertising; we've always been told to use Facebook's targeting tools which are apparently really easy. You'd need to make sure the maths made sense for you though - there's some info here on how much you'd likely be spending to get a user to pay for you game: https://www.bluecorona.com/blog/how-much-facebook-advertising-costs
Good luck!