Visas are available on arrival at the international airport in Kathmandu and at all land border crossings that are open to foreigners. Your passport must be valid for at least six months since the moment of arrival and you will need a whole free page for your visa.
You also need to have passport photos to hand and be able to pay the visa fee in foreign currency (some crossings insist on receiving payment in US dollars). All foreigners, except Indians, must have a visa. Nepali embassies and consulates overseas issue visas without any difficulties.
A Nepali visa is valid for entry for three to six months from the date of issue. Citizens of South Asian countries (except India) and China need visas. However, if you are only entering the country once in a calendar year then these are free.