By air: Nepal’s only international airport is in Kathmandu. Getting a direct flight could prove difficult so you may have to take two or three flights to get there.
By bus: There are four overland routes into Nepal via bus which are frequently used by travellers. Three come from India and one from Tibet.
If you have a set itinerary, flying may be the only way to stick to it as the road network in Nepal leaves a lot to be desired. This means buses are largely unreliable. There is one train line operating in Nepal for passengers between Janakpur and Jaynagar in India.