Iam into horizontal honda engines . Iwant to learn how to rebuild them .how do i start ?

like the gentleman above has said, get ready to do a lot of reading. Start by looking at the parts diagrams on places like bikebandit for those engines, get to know them. Buy one from a salvage yard and tear it down-nothing like working with your arms up to your elbows in grease and grim. Rebuild it and get it running. Nothing will be more satisfying.