How do tax write offs work?

My sister says that she has a consulting job on the side, and that when she flies to visit family or whatever she "writes it off" on her taxes. I donated 80 bucks to a charity and the guy said it was tax deductible, but when i filled out my taxes there was no place for me to enter in that information. I would like to know how tax write offs work, if there are monetary restrictions (do i have to make a certain amount to qualify?), and step by step (if possible) how to fill out the forms to get my money's worth. Thank you!