You have two options flight or overland to get into Nepal. Nepal is landlocked country so it is only possible to go via airways or by land transport.
In order to fly directly to Nepal from your home country, Tribhuvan International Airport (TIA) in Kathmandu is the only international airport in Nepal. TIA has direct air link with Osaka, Shanghai, London, Frankfurt, Hongkong, Singapore, Bangkok, Delhi, Dubai, Bombay and Calcutta.
Nepal Airlines (NAC), Lufthansa, Air India, Singapore Airlines, Thai are the airlines that carry most of the foreign travelers into Kathmandu. Korean air, Dragon air, Air China, air Arabia, Kingfisher, fly Dubai are other airlines for Nepal flight.
If you are traveling to Nepal from India or Tibet (Autonomous state of China) and you time and enthusiasm, traveling overland is an another option. Some British travel companies are directly operating overland tour services from Lodon to Kathmandu via Australia but it takes long time and costly.