I've been playing chess for a long time but don't seem to know how to take the game from playing on a board to playing on the computer. Maybe someone can help.

It is a simple matter of experience. You will get used to it soon enough. So much so in fact that your Over The Board chess might even suffer from getting acustomed to seeing the 2-d pieces.

Have a 3-d board set up nearby & play your 2-d moves there also.