I think my dad is cheating on my mum i dont know what to do help?

You need to confront him with the information you found. He needs to know that what he's doing is completely unacceptable and that you know what he's doing. Also, your mom deserves to know so you need to give him an ultimatum-either he tells her or you do. He needs to be the one to end his affair and tell your mom, but your mom needs to know either way. What a crummy way to find out-your mom could have easily found out that way! Make sure he knows how you feel about it and how disappointed you are in his behavior.