Kathmandu the capital city of Nepal is connected by air with Bangkok, Honkong, Shanghai, Lhasa, Chengdu, Osaka, Singapore, Dubai, Doha, Abudhabi, Karachi, Dhaka and the Indian cities of Delhi, Mumbai, Bangalore, Kolkata and Varanasi. Royal Nepal Airlines is the national flag carrier. Thai Airways operates daily flights between Bangkok and Kathmandu. Other airlines are flying in and out of Kathmandu are Qatar Airlines, Gulf Airways, Indian Airlines, Jet Airways, Sahara Airlines, Druk Air, Air China, Biman Bangladesh and Pakistan International Airlines. Austrian Airlines and Martin Air seasonally connect Kathmandu with Vienna and Amsterdam, respectively.
As Nepal is surrounded from three sides by India, and if you are traveling in India prior to Nepal, you can see if its possible to travel to Nepal via road. The border cities of Nepal are popular entry points used by tourists to enter Nepal on a daily basis.
Kakarvitta is Nepal's eastern border town. People traveling in Sikkim and Darjeeling can enter Nepal via Kakarvitta. Bhadrapur airport of Eastern lies just half an hour drive from Kakarvitta and has regular flights operated by domestic airlines of Nepal. In the Indian side, Bagdogra airport lies in one hour driving distance from Kakarvitta border.
Gorakhapur Railway Station is located in about 2 hours driving distance from Sunauli and is one of the most used entry points by travelers. Gorakphur has railway connection with major Indian cities including Delhi and Mumbai. People who are already in Varanasi also enter Nepal via Sunauli. Its 8-9 hours drive from Varanasi to Sunauli.
Kodari is the border town used by travelers to travel overland between Nepal and Tibet. From Kathmandu, Kodari is 113 km and is a journey of 4 hours. From Kodari, Lhasa -- Tibet's capital city -- is a 5 days overland journey, driving 6 hours in average.