First off welcome to the forum. Even French people are welcome
Ok, you have your concept, now it's time to start marketing.
Do you have a website? if not, that would be a great place to start then you can start SEO on that website, also if the game is yet to be completed and released into the open market, a website would be a great place to start generating the buzz about the game bu leaving little teasers and tantalizing questions.
Also you have the option to add a subscription box so people (that could be potential buyers/users of the game) can stay up to date with the latest going on and where you are with your development.
You have the option of a blog too, which might be just as good seeing as you can update that easier. If you frequent to game forums, placing a link to your blog in your signature would be a great way to entice members to go.
You could use AdWords as suggested but in the online games market your looking at a massive amount of money. Then again it depends on what your budget is etc etc.
Buying an email list is another option, one that is targeted at users that play online games.
That's just a few things but there is a lot more.
KP