If you'd like to write to Josh, the best way to go about it is to contact him through his fan mail address, at:
137 N. Larchmont Blvd. #622
Los Angeles, Ca. 90004
You can use the form here to send an email message to Josh.*OR*
You can leave a message for Josh on his message board at www.joshgroban.com in the "Josh, Please Read This" thread. Josh DOES read the message boards and posts there whenever he can.
Whichever method you use, please don't expect a personal reply. Josh is great about reading his messages and mail as much as he can, but he's extremely busy and there's no way he could answer everyone who writes to him! He does have an assistant who sometimes sends responses on Josh's behalf, however. PLEASE DO NOT SEND GIFTS OR PACKAGES through the mail! This is a request from Josh and his family, so please respect their wishes.