To enter Nepal, a Nepalese visa is needed(except for Indians). You can easily get your Nepal visa in advance from the embassies of Nepal in your country.
Also, a visa for Nepal is easily available upon arrival at Nepal's only international airport in Kathmandu. Nepal's 'On Arrival' visa procedure(around 45mins) is very quick and simple. You can expect some queues during peak tourist season. If you wish to skip those queues, you can also consider getting a visa from Nepalese Diplomatic Missions stationed abroad before your arrival. Be noted that tourists from Nigeria, Ghana, Zimbabwe, Swaziland, Cameroon, Somalia, Liberia, Ethiopia, Iraq, Palestine, Afghanistan, Syria, and Refugees with travel documents are not given visas on arrival( need to acquire Visa prior to their arrival). On Arrival Visa Fee at Entry Points:15 Days-30 USD; 30 Days-50 USD; 90 Days-125 USD.