You can book this through pretty much any travel agency you find in Thailand, in Chiang Mai or Bangkok. The trek is done in the jungles of Chiang Mai which you can get to via 14 hour train ride for about $15 from Bangkok. The price of the trek varies between around $100-200 for the 3 day trek (which I definitely recommend). Food, gear, and an awesome guide are all included. Each place you camp at has a cooler and you can buy soda, beer and snacks. The food they cook on the trip is super tasty. I actually became really good friends with my guide so leave a comment below if you want me to put you in touch with him.

On the 3 day trek you will ride elephants and hike to a waterfall and then camp in a small village on Day 1. Day 2 you will hike to another waterfall and camp nearby, seeing lots of rice patties and beautiful landscape all along the way. Day 3 you will hike out of the jungle where a truck will pick you up and take you to the river to ride on bamboo rafts. So sick!

Here is one suggestion for a guided trek but search around and find one that fits your budget and needs - there are plenty to choose from.