Sing the first note that comes naturally or feels comfortable to you. Either that or whatever note you think you remember the most often and pick that as your reference note.
Verify that note with the pitch below and determine which note it is. Once you've done that you are going to use this note to sing up half steps to whatever note is played in the game.
Over time this will become quicker if you do it every day and pretty soon, you will recognize all of the pitches rather than singing halfsteps up or down everytime. Have fun!