By Air:
Tribhuvan International Airport is the only international airport in Nepal. The important airlines that operate flight to Kathmandu are Indian Airlines, Thai International, Bangladesh Biman, China Southwest Airlines, Druk Air, Qatar Airways, PIA- Pakistan Airlines, Gulf Air, Sahara Ailrlines, Jet Air, and Silk air. The national carrier - Nepal Airlines Corporation (NAC), operates flights to Europe and Japan as well as the regional destinations.
You can fly to Kathmandu via Bangkok, Hong Kong , India and Singapore. There are daily flights to Kathmandu from bankgkok.
If you are in India, you can fly to Nepal from Delhi, Mumbai, Calcutta, Banglore and Varanasi. And There is the fantastic flight from Lhasa to Kathmandu for three days a week (Saturdays, Tuesday and Thrusday) operated by China Southwest Airlines. You can also fly Druk Air from Paro in Bhutan, or take a flight from Dhaka, Bangladesh.
From Europe and Middle East
Qatar Airways and Gulf Air operates daily flights to Kathmandu from Doha and Dubai. Pakistan International, Bangladesh Biman and Aeroflot have one-airline service from Europe to Kathmandu. The other option is the charter companies.
From North America
you could fly to Nepal via India or alternatively via Bangkok or Hong Kong. RNAC's Osaka flight makes good connections with North American flights.
From Australia and New Zealand
Look for routes via Singapore, Hong Kong or Bangkok.
By Land:
There are some entry points into Nepal by land open to foreigners, from which six are from India and two from Tibet.
Via India
The entry points from India to Nepal are Mahendranagar, Dhangadhi, Nepalgunj, Sunali, Birganj and Kakarbhitta so you can get on the direct buses to the Nepal border from Delhi, Varanasi, Calcutta, Patna and Darjeeling and From the border, you can get on Nepali buses to Kathmandu/ Pokhara.
Via Tibet
you can cross the border into Nepal from Tibet via Kodari.