Always remember, your current dog needs to be treated as the top dog - for now at least.  This is "his" home, so he needs to feel secure in his position.  To introduce your dog to the new dog, pick a neutral place such as a park, and ask a friend for help.  Have your friend walk the new dog a good distance away.  Allow the dogs to see each other. Walk casually toward each other a little closer. 

Begin to talk happily to each other, as if this is "no big deal."  Do not get close enough for the dogs to touch.  Begin to walk parallel to each other, again talking pleasantly.  Continue until the dogs seem comfortable.  Now allow them to get close enough to smell each other.  If all goes well, let them explore and play with each other.

If you feel the dogs need a bit more time, keep them from being close enough for contact  and repeat this process until the two seem comfortable together.  You can pet the new dog and allow your current dog to smell your hand and visa versa to help further the introduction phase.

Next, meet with your friend and the new dog outside of your house and repeat the pattern.  Keep them far apart at first - this is important.  Continue with the dogs walking parallel until they are comfortable enough to finally meet.

If you have an older dog and are introducing a puppy, never leave the puppy alone with the older dog.  If the older dog snaps or  "complains," the puppy may think this is a game and continue to bother the older dog - which can lead to dangerous consequences for the puppy.