Getting another dog? Here are some links with advice on how to go about introductions.
In general trainers advise introducing the dogs first on *neutral territory*, not bringing dog two straight into dog one's territory of the home. So do the introductions in the local park with both on leads and take them for a walk, or maybe at a friend's house or a friend's garden.
Let the dogs get to know one another but keep an eye on interactions and know what to watch for if one is getting stressed. They will likely have some minor scuffles as they establish who fits where in the household, and this is normal too. I would NOT let them 'fight it out' however. Remember YOU run the house and set the standards for polite behaviour with your dogs and fighting is not acceptable behaviour. So anything beyond a minor scuffle -- eg some growling and a snap where they don't actually touch each other -- should be broken up (put the dogs in two separate rooms for time outs for 10 minutes or so til, everyone calms down, then try cautiously letting them mix again). Note that some trainers believe you should "let them fight it out" but none of the trainers whose opinions I respect agree with this approach, which can set dangerous precedents for what is allowable behaviour and also cause serious injury -- even to you if you try to break up the fight. It is your job to watch for any sign that a dog is getting stressed enough to lash out and there are many signals that pressure is rising. Dogs for the most part do not want to fight and will give plenty of warning before things reach that stage, most of the time.
Don't leave two dogs that barely know each other alone together. You'll need to keep them in separate rooms for example if you are going out. Generally I feel it takes at least about two weeks for you to gauge whether they are OK to be left alone, and this is only with exemplary behaviour during that time, with no flashpoints.
Feed a good distance apart or better yet crate them for feeding. Food (and treats and chews) are very often the flashpoints between a new and resident dog so your best approach is -- as always for the responsible owner -- not to put the dogs in a situation where they are allowed to act in a way that isn't acceptable (in other words, always try to set your dogs up for success, not failure, so they are from the very start praised for making RIGHT choices not punished for making WRONG choices). I generally feed a new dog at least 6 feet away from existing dogs in the house and often feed the new one in a different room, like the bathroom.
Don't overly fuss over the new dog especially not to the point where you might give less attention to the resident.
Expect some initial jealousy and for the resident dog to not be too happy with the situation. Puppies tend to be accepted more quickly and readily than a new adult dog. If your dogs are not spayed and neutered, you will seriously want to consider this preferably BEFORE the second (or third... etc) dog is added. Unneutered pets run a much higher risk of fighting and many feel this is even more likely with two unspayed females than it is with two unneutered males. Be aware that puppies as young as FOUR MONTHS can (though rarely) become sexually mature enough to either impregnate or get pregnant so this is another reason to have the resident pet neutered already.
Most dogs will become great friends with time -- but be prepared to find that as with people not all dogs become great buddies. It depends on personalities, time, sometimes your attitude, their environment, etc. Most dogs however do become great pals and I strongly believe having a canine companion for your dogs gives them a much richer and psychologically healthier life than being an only dog.
Excellent and detailed consideration on introducing a new dog or cat to a resident dog or cat: