If you already have a pet that is used to being in your home you should realize that it may take some adjustment time before they are ready to fully accept the new addition.
When you first bring the new puppy home do not trust any other pets completely until time has passed and you are sure they will be safe together. You can begin by placing the puppy in the crate in an area where the other pet often frequents. Let them smell each other through the bars. This separation will accomplish several things. First of all it will of course keep the youngest one safe. Secondly it will allow you some time to assess the situation in order to see what the reaction will be from the other pet. Thirdly it gives you some time to pay special attention to your old friend which will assure them that they are still #1 in your heart.
Jealousy can be a big problem if you don't stop to realize what you are doing. A brand new puppy is such an exciting thing that it is easy to lavish it with alot of love and attention and hugs and kisses. If your old friend is laying at your feet watching, he may make the connection that this puppy is moving in on his cuddle time! Dogs can often be much smarter and more sensitive that you would originally think. Be sure to spend extra time with your pet so they know that you still love them also.
Other dogs welcome new pups with excitement. A new playmate can be lots of fun! But, even in this case beware around food bowls, bones and toys. Just because they like to play or nap together does not mean they will be happy to share their belongings or food!
If your older pet is not to happy with your decision to adopt - give it sometime. They will generally adjust fine if you do it with care.