The only language I know for Plan 9 is C, and that doesn't seem ideal for web development. I also understand that it doesn't run apache or mysql either.
What is the best way to start coding web apps on Plan 9?
Check out Kenji Arisawa's Pegasus (paper) webserver for Plan 9.
Plan 9 may have a reputation for being C-only, but several langauges, including Scheme, Ruby, Python, and Perl have been ported. Check out the Contrib Index for the code.
Finally, start reading the Plan 9 white papers so that you can understand its philosophy. If you want to do net-related things, the file protocol 9p is essential.