You can apply online for a VISA through Department of Immigration prior to your arrival. However, as a tourist it is easiest to attain a 15, 30 or 90-day VISA on your arrival at Tribhuvan International Airport.
Tip: Use multiple-entry visa if you are planning to travel to Tibet, Bhutan or India. You can change your visa from single-entry visa to a multiple entry bias at Central Immigration Office at Kathmandu for as low as US$25.
Read more at : https://www.coretreks.com/faqs on Visa Faqs section.