Determine the type of saxophone. Baritone, tenor, alto or soprano each need a different size. The manufacturer will indicate the instrument on the product container.
Judge your skill level. Beginners usually use a softer reed (size 1, 2 or possibly 3). The longer you play the harder the reed because your embouchure gets stronger.
Select your material. Cane reeds are used by most professional musicians. You can also use plastic-coated reeds and even fiberglass reeds. The fiberglass reeds can last for many years, but sometimes produce a thin sound.
Adjust your reed strength based on the type of sound you want. Softer reeds play the lower notes easily, but often struggle with the upper ranges and don't always give you a clean sound, while hard reeds are louder and can hit the high notes with less effort.