What Are The Reasons Behind Bad Behavior In Children?

I have a 5 yr old girl and I don t know what to do with her. She has a smart sassy mouth and never stops. She talks back and refuses to pick up her toys. She always gives problems in the morning when getting up and getting ready for school. She always tells me to go away, I don t like you, your mean,etc. it s impossible to talk to her or anything.