Either chose a name that's relevant to the game, something made up of relatable keywords, or just make up a word that sounds good, or maybe a word that sounds like the game. Like Tetris, or Pong. Simple words are probably taken, but you can always make up your own, or combine names and words with the game setting, like Mario Tower Defense. I find making names the easy part, it's making the games that's hard. :) I'd give suggestions if you gave more information on the game, but maybe that's not something you want to give out yet? Good luck!