Unless travelling overland from India, most visitors to Nepal find themselves arriving into Kathmandu's Tribhuvan Airport to the East of the city. From here it is a short journey into the centre of the city, and that one short bus ride is usually enough to put people off using public transport in Nepal again. There are few paved roads within Nepal, and the buses that operate along them are overcrowded dirty and uncomfortable, not to mention painfully slow, but those who persevere with them will find that they are a sociable way of travelling, and that the shortcomings are part of their charm. It is not possible to arrange car hire in Nepal, although locals will act as drivers and guides if you hire them.
The best way to get around Nepal is to walk, the terrain is rough, and the mountains are steep, but to really see the country means to get out into it and explore away from the roads.