ORIGINAL: BGfisher
If you are just starting out then measure your wingspan and divide it by 2.5. This will get you closer than any other method. And get a bow that is adjustable till you have about 20 years of experience and know exactly what your draw length is.
In shorter draw lengths, the other wing span method often works better. That is subtract 15 from your wing span then divide by 2. This gives shorter DL's for archers with less than 30" DL.
Both methods have to be modified for body type. Wing span calculation gives me a 27" or 27.5" DL. However, I have a larger chest than most so I shoot most comfortably and accurately at 26.5".
After youset up by wing span calculations, get a photo of yourself from the side at full draw. The nock of your arrow should be directly below your eye.
If you shoot seriously and practice year round, you will probably change your DL, but by then you will understand how DL affects your shooting.
The best way for a beginner to get their DL properly set is to go to a good pro shop and let them do it.