Competitors analysis is no trivial task by any means. The market has a lot of titles. What I did was go through SteamPy, Steam Stats and other similar sites. Knowing what game you are making you can look at player stats, geography distribution, game sales. Going through webpage design , layout of competitors is common practice in non game world. Also check social media pages of competitors, how they respond to comments, what they post/reposts, how they approach players. I daily read press, look at comments to other games pages, what gamers like, what dislike.
If you plan to do Kickstarter campaign then look at successful and failed titles, pay attention to their updates and comments.
Then I suggest using one of social media analytics tools, there are a lot of them, and some of them offer free trial. You can analyze popular pages or target specific game page.
I will write more about marketing later. If you have any questions fell free to send mew a private message.