Strings can be a hassle. Here are some things you need to know. Some places sell strings by AMO length so if you have a 60" AMO bow they sell you a 60" AMO string. Most dealers sell by actual length and the best way to determine the actual length is to measure it while the bow is strung. As Vermonster said, the string is usually 4" shorter than the AMO length, but not always. In your case I would bet the 56" length will work.
There are basically two types of string material: Dacron, and all the low stretch materials. B-50 and B-500 are both Dacron strings. The more common low stretch materials are Fast Flight, TS+, Dynaflight 97, and B450+. There are several other varieties.
You are ALWAYS safe with a Dacron string. If a bow was not made for Fast Flight, it could break if you use any of the low stretch materials.
The most common types of bow string are the endless loop and the flemish twist. Either will work on your bow. It is the material that is most important, not the style of string.