Whenever you bring a new pet into the household there will definitely be a big adjustment period as your older pet defends the family against what he or she sees as an intruder. Here is how you can make the adjustment as easy as possible.

Scent
You first want the pets to get used to each other's scent. So place your new pet in a room (and even in a kennel in the room) with the door closed so that your other pet gets familiar with the scent.

Introduction
Introduce the pets in a room where you can monitor the initial meeting. You should expect some barking or hissing when they first meet, and this is normal. But be ready to react if one of the pets becomes aggressive towards the other. You might want to give each pet treats as rewards for a positive meeting, and this will help them both associate the other with positive feelings. You should keep your kids out of the room for the initial meeting incase you need to step in, it is natural for a child to feel they need to protect the older pet, and you don't want your child to accidentily get in the way.

Extra attention
Make sure your older pet gets plenty of attention, such as extra grooming or playtime. This will help prevent your pet from feeling jealous of the attention a new pet inevitably gets.

Never leave them alone
Do not leave your two pets alone together unless you know they will get along fine. This might mean you need to separate them when you are out of the house.