To enter Nepal on a tourist visa, you must pay in cash upon entry, so you will need to carry the funds on your person. You will require at least six months validity and a full blank page in your passport when you enter Nepal, as well as a passport-sized photo. There are booths for taking these pictures at Kathmandu airport, but if you are coming by another method of transport you should bring this photo with you (and as you will need these photos for your trekking permit, you should bring them anyway). Current prices for visas are as follows:

15 Days – 25 USD
30 Days – 40 USD
90 Days – 100 USD

Note that the equivalent prices can be paid in currencies other than USD. For full details on getting your visa to Nepal, and to check whether your country has exceptions, you can visit this website.

Another provision of a visa system that you need to know is about the Gratis (Free) Visa for 30 days is available only for nationals of SAARC countries. However, for extension of visa for SAARC nationals, the rule is same as that of other nationals. It is to be noted that Indian nationals do not require a visa to enter into Nepal.
When it comes to visa extension, tourists are allowed to stay for a maximum of 150 days in a visa year (Jan 1 to Dec 31) extending the visa at the rate of 2 US $ per day. More precisely, a minimum amount of 30 US$ has to be paid for a period of 15 days or less.