You can get your visa when you arrive at the Tribhuvan International Airport in Kathmandu, or at border entry points in Kakadvitta, Birgunj, Bhairahawa, Nepalgunj, Gaddachowki on Nepal-India border and Kodari on Nepal-China border. You can also obtain your visa at the nearest Nepal Embassy or Diplomatic Mission. You can also get your visa in the Department of Immigration, Kalikasthan, in Kathmandu. You need to bring your passport valid for at least 6 more months and a passport photo (with a light background). It costs 1275 rupees for a Tourist visa with multiple entry for 15 days, 2050 rupees for 30 days, and 5100 rupees for 90 days.
You can’t work in Nepal with a tourist Visa, even as a volunteer.