HOW TO FIND IT:
Locate the "keystone" group of four stars of Pi, Eta, Zeta, and Epsilon in Hercules.
Scan between Eta and Zeta. It will be easy. In fact, you ought to be able to see it with the unaided eye.
WHAT TO LOOK FOR:
If M13 is not the best globular cluster in Messier's list, let me know what is.
Look for a disk of light with a bright center. The observer should be able to resolve individual with a small telescope.
Study the nucleus. Photographs won't show this, because they overexpose the center in order to photograph the surrounding stars, but there are variations of brightness in the nucleus, reflecting some areas of the cluster's nucleus that are not as dense with stars. See if you can discern any lanes or star chains. You may be able to see up to three that cut across the nucleus.
While you're in the neighborhood, look half a degree northeast to find a small spiral galaxy, NGC 6207. You will not be able to see it with binoculars or a very small telescope.