Tribhuvan International Airport is the only International airport in Nepal and therefore will most likely be your entry point into the country. The airport lies 6km from the city centre and is a 25 minute drive to the popular tourist area of Thamel.
Accessing Nepal by land can be done via India or China:
From China: Explorers make their way from Kerung, or Tatopani (Although the Tatopani entry point is currently closed).
From India: There are multiple access points to enter Nepal depending on the direction of travel (Sunauli, Raxaul, Kakarvitta etc.)