A bit of play-fighting between two cats who have just met one another is nothing to be worried about - it's normal for cats to chase each other and wrestle around the house (and this phase can even last as long as a few months). If the cats are hissing, claws are out, and if they're growling or crying - these are all signs of distress that should not be ignored. Try your best to break up these fights but don't get caught in the crossfire: the best way to break up a catfight is by stomping or clapping loudly.
Equal attention
Try not to play favourites when introducing a second cat. Both pets will need a little extra love, so be sure you're giving them plenty of individual attention; spend extra time playing with them separately if possible.
Separate litter boxes
Cats are territorial creatures, particularly when it comes to toilet behaviour. Introducing multiple litter boxes means they won't fight over them and each will feel he or she has an adequate amount of personal space in the house.
Give rewards for good behaviour
Positive reinforcement can go a long way - kind of like training a dog. When both cats manage to hang out successfully without any scuffles or trouble, give each one a special treat to show their good behaviour is appreciated. This could be a favourite toy or a special edible treat.