Most bluegrass players use either phosphor bronze or 80/20 brass strings. Buy phosphor bronze strings to obtain the best mixture of sound and durability. Phosphor bronze strings are made from a combination of copper and tin, providing both sound quality and resistance to wear and tear. Brass strings are also suitable for bluegrass acoustic guitar, but they are less durable than phosphor bronze. Avoid nylon strings, as these are not suitable for bluegrass guitar music.
Buy medium gauge acoustic guitar strings for bluegrass. Generally, bluegrass players use strings of the gauges 12 to 56. Guitar string gauge is measured in thousandths of an inch, so this means that a larger number means a thicker string. Use the higher three strings from a light gauge set of strings, and the lower three strings from a medium gauge set for the perfect bluegrass sound. This combines ease of playability on the higher strings with the full, louder sound associated with heavier gauge strings. You can choose to use other gauges of strings after you've become familiar with the feeling of traditional bluegrass gauge strings, but these are generally thought to have the best sound for the genre.
Use round wound strings for bluegrass playing. The thickest four strings of any set of acoustic guitar strings are made from a thin core with a certain material wrapped around it. Round wound strings are wrapped in a rounded wire, which results in the ridges you may have noticed on the thicker strings on your guitar. Round wound strings produce a clearer and louder sound than flat wound or ground wound guitar strings, and this is preferable for bluegrass style.
Many string manufacturers make strings which are specifically labeled as being suitable for bluegrass playing. Buy strings labeled "bluegrass" for the easiest method of finding strings suitable for the genre. Manufacturers such as Martin, D' Addario and John Pearse make bluegrass-specific guitar strings. The D' Addario and John Pearse strings are 12 to 56 gauge, and Martin's are 16 to 56 in gauge. Martin's strings are nickel wound, D' Addario's are 80/20 bronze and John Pearse's are phosphor bronze.