Advice Would Be Greatly Appreciated

My lovely baby girl Ashley is sometimes not so lovely. I will be very honest here and say she is a brat. I love her to death, but don't know what to do. She talks back, hits her brother, screams etc... I discipline, I spank and they don't seem to have an affect on her. Has anyone else experienced a problem with their child like this. I don't want her acting like this. I don't want her being the type of child that misbehaves and people don't want to be around her. I've seen children like that and it's awful. I know this is my responsibility and that is why I'm reaching out and asking for help. I would appreciate any type of advice or direction you could provide. Thanks and God bless!