All visitors to Bhutan other than those from India, Bangladesh and the Maldives require a visa. Visas are processed by licensed Bhutan tour operators and issued only to visitors on all-inclusive prepaid tours. The formalities are done in Bhutan, for which you will need to provide a copy of the photo page in your passport. A visa clearance letter will be issued once the payment for your tour (including a US$40 visa fee) is received and a visa is stamped into your passport on arrival.
The Tourism Council of Bhutan web site has details of the visa process here.
There is a directory of licensed tour operators on the same site here. You can deal direct or through an overseas travel agent.