How to help my kids get along?

My son is 5 and my daughter is 3... and they fight ALL THE TIME, every second of everyday, legit from the time they awake till the time they go to sleep. My son is at school for the most part of the day and when I get home from work with him the fighting starts. Im constantly trying to correct their behavior towards one another or help them to try to get along but nothing is working. They say they hate each other and it hurts me. Like why can't they get along ? Is this normal and just going to get worse ? I'm hopeless