I assume this is the first time they have played or that they are new players?

The number 1 piece of advice I give DMs who are DMing new players is to establish a few strict rules off the bat, things like "no evil characters", "no comedy characters", but most importantly "PCs are not allowed to attack one another or steal from one another."

Yes I'll probably get harangued for imposing such severe limits on my players, but I think it is helpful for new players to have those limits because D&D is already a game without limits. New players are going to want to test those limits, and it is best they test the limits of the world rather than the limits of the characters around them. The way I justify this last rule (and I tell this directly to the players) is that, at the end of the day, your character has to have a reason for wanting to be in the party. For instance if you design a character who hates all Dwarves and you are travelling with a group of Dwarves, that is fine, but you need to have an in character reason for letting your prejudices slide.

If you have characters who are constantly fighting, just literally say to them: "Why are you guys still a party?". Get them to explain themselves. If they find an explanation that works, maybe that can be an interesting story thing (e.g. "two rivals who are constantly trying to outdo one another"), but if not, just tell them the party disbands, and ask them to draw up new characters.