You have two basic choices for a sleeping bag, down or synthetic. Both have advantages and disadvantages. A synthetic bag will look attractive for its lower price and ability to remain somewhat warm if you get in an emergency situation where your bag is wet. Its disadvantage is its weight, bulk and the accelerated pace at which its filing breaks down. A good quality synthetic bag will last you the length of the trail. A low quality synthetic bag will probably have to be replaced before the end of the trail.
Down is a superior fill in that a down bag will typically be lighter in weight, much more compressible and have a long service life. The disadvantages with a down bag are price which tends to be more expensive and some people may have an allergy to goose feathers. A high quality down bag such as one from Western Mountaineering, if properly cared for, will have a significantly longer life than a synthetic bag.
Most hikers on the AT will be carrying a 20 degree to 30 degree bag. This keeps you protected in the cooler months and then you can unzip the sleeping bag and use it as a blanket in the summer. The thing to remember is temperature ratings are suggestive. A lot goes into determing how warm you are going to be in a bag. Its quality, your level of fatigue, even down to what you have had to eat that day. Get a good sleeping pad to add to your comfort and the bags ability to keep you warm. Heat loss through the ground is one of the biggest areas of heat loss around the body in a sleeping bag. A sleeping pads helps to prevent this.