This is my first time traveling to Nepal in US passport. I will be there for about 20 days. Can someone walk me through the steps. I'm feeling a bit anxy.

Requirements for Entry:

Passport valid for at least six months
Visa
Tourist Visas: Either apply for a visa at a Nepalese embassy or consulate before traveling, or purchase a tourist visa upon arrival at the Tribhuvan International Airport (Kathmandu) or at the following land border points of entry:

Kakarvitta, Jhapa District (Eastern Nepal)
Birgunj, Parsa District (Central Nepal)
Kodari, Sindhupalchowk District (Northern Border– for group tourists only)
Belahia, Bhairahawa (Rupandehi District, Western Nepal)
Jamunaha, Nepalgunj (Banke District, Mid-Western Nepal)
Mohana, Dhangadhi (Kailali District, Far Western Nepal)
Gadda Chauki, Mahendranagar (Kanchanpur District, Far Western Nepal)
You may request:

15-day multiple-entry tourist visa (25 USD)
1-month multiple-entry tourist visa (40 USD)
3-month multiple-entry tourist visa (100 USD)
Tourists may stay no more than 150 days in any given calendar year.

See the Embassy of Nepal’s website for further visa information.

Thanks maxpayne. Do you know how much cash (dollars) can you travel with? Will I need to reveal that once I'm at TIA

google it bro, those things are easy to find.