You can either apply Nepal visa beforehand or get one upon arrival at the Kathmandu airport. The visa process is quite easy and simple. You should apply for tourist visa.
Visa Fee in Kathmandu airport
15 Days – 30 USD
30 Days – 50 USD
90 Days – 125 USD

Visa Extension Fee 
15 days – 45 USD (3 USD for additional days)
In the case of delay less than 150 days additional USD 5 per day as late fine.

For more information about Nepal visa;