Can I use games I already have and modify them?

Yes, you can build your own version of a board game like Clue or Monopoly, program a game similar to Breakout or Angry Birds, or create a campaign for an existing role-playing game.  However, be sure to change the game elements (such as the artwork and the game theme or story) so that it becomes your own game and not just a copy of an existing game.  It is also important for you to change the game rules and then bring together some friends to play the game with the changed rules so that you gain an understanding of game mechanics and what makes a game fun to play.