After building a good foundation on your horse with groundwork (*see bottom of page for references), your horse may be ready to carry a rider.
For a young horse, it takes a while for him to be able to move and carry a rider. He needs to get used to it; to build up his muscles. Take it easy at first. Make the sessions short and rewarding.
The following are some pictures of a young horse being started by David Lee Archer (website):
Haltering with a rope halter.
Pre-ride check: Ask your horse to move his hindquarters, both directions.