Jerry,
Ron and Hannes have given you some excellent information already.
The Germans began testing them in mid-1917 and began general issue late that year. It was still considered optional equipment.
The Allies did not encourage the use of parachutes and apparently squelched reports of their usefulness. Top brass apparently thought a pilot would be more prone to stay with a damaged plane and bring it in if he had no chute. (As if a flaming object made of wood and cloth had any salvage value). There seemed to be no thought of the pilot's value. Furthermore, it was thought to be a device for "sissy's". However, some Allied pilots did buy their own parachutes through private sources.
In addition to
Ernst Udet, parachutes were used successfully by
Josef Jacobs (twice) and
Paul Bäumer--the three top German aces to survive the War. The result was that these highly-experienced men were able to continue shooting down Allied aircraft because they used these "sissy" devices. There were many other cases as well.
Best Wishes,
Gary