Hi dimitriosfl I'm not an expert in that issue, and I'm pretty noob in all this, but I'm think your game from the moment of publishing automatically get some local new top list. After this moment all depends your downloads. Anyway after a month in the same way you get out from top new (I think it's depend on number of you update also (updates prolong your "new" period but I'm not shure)). The target is to reach a decent number of downloads to get "Not NEW" but "Really" top in any category. You have to know that any Markets Top in any category is local to your country region only etc.
You have a good and free (for basic function) analytical tools like AppBrain, AppEnnie etc. You can check all your games ranks for free.