I need help to see how to get a formula for this.

if a person signed on at 10:22 am and finished at 4:45, how can i get the
excel sheet to tell me how much time was used on the project? I am a real
novice, so really simple explainations would be greatly appreciated. I
checked all of the ones available on here on the online discussion, but was
lost all the way through.
simply stupid I guess.
thanks
Sandra