This one should be obvious, without participants you don’t have a tournament. But there is more to it than just getting players to pay the entry fee. Make sure to find and focus on a target demographic to ensure that your tournament is filled with players that will be more inclined to make donations to the cause that you are supporting.
But just because the players showed up to the course the day of the event doesn't mean that your work is done there. Giving your players the best possible experience is your top priority once the tournament gets started. Not only is a happy participant more inclined to donate to the charity but they're more inclined to come back to the tournament and they might even bring some friends with them.