I have already set my koji server and every component is work OK.
But not have any document mention howto init the buildroot?(or use mock
It's not entirely clear what you're asking, but here goes...
mock is used by koji-builder to create buildroots. In normal operation,
you submit a build to the system, the builders pick it up and use mock
to create a buildroot and run the build.
If, on the other hand, you want to run mock manually against the repos
that koji maintains (not normal operation, but useful nonetheless), then
you should just be able to generate a mock config with the 'koji
mock-config' command. You can use that config with a manual run of mock.
Sometimes the config may need a little tweaking.
If this doesn't answer your question, then perhaps you could provide a
bit more context.