Trekking in Nepal
The simplest form of trekking is to slip a few essentials into your daypack with a map and to start hiking from Lukla or Pokhara trailheads. You can manage well enough following the signs and staying at lodges, and you'll pay roughly US$10 per day. For a much more enriching and secure experience you can hire a guide or at least a porter who knows his way among the trails. He will give you much deeper insight into culture and customs, as well as keeping you out of trouble, and for only another US$10 per day, plus incidentals. If you're with a larger group and you really do crave the smell of fresh yak, you can hire a full complement of porters and a guide to lead you through the best routes and to set up camp for you. The costs do go up quite a bit, but so does the quality of the adventure. Remember that as you wander higher in elevation you'll need to spend time acclimatizing, and what a dread it would be to spend all that time alone.
In the early days, as Nepal opened its borders to Westerners (a.k.a. “before the white monkeys”), organized trekking parties were a novel curiosity for the Nepalese. In the 60's and 70's the idea began to catch on and many more people sought out the adventure. At that time there around 60 tour groups leading trekking tours through Nepal, but now it has gotten so popular with Westerners that there are currently over 400 companies. That makes for good competition, but make sure you find an outfit with proper credentials. There are also niche services available, such as treks for conservation (picking up rubbish along the trails) and birdwatching.