Bus from Chiang Mai takes five hours. There is an airport but for a number of years there have been no flights. Bus station is 4 Km west of town, and you will need to take a moto taxi to town.
Bai Fern Guest House is a nice place to stay, single fan room was 150 baht when I stayed there last spring. Free wi-fi. Two excellent restaurants where you can meet up with other falang volunteers are the Aiya (across the street from the Bai Fern) and Casa Mia--it may sound Italian, but they have the best Burmese curry and tea leaf salad you ever ate. Mains cost 30 baht at both places.
Also Thaime's bar has weekly quiz night (I forget what day) where all the NGO'ers team up in a game of cultural trivial pursuit. It is great fun.
A word of caution in advance of your host NGO hopefully sharing the same warning: Mae Sot is crawling with Burmese SPDC spies. They speak perfect unaccented English and will try to strike up a conversation with you about your travels and gently guide it to your activities in Mae Sot. They will also sit in the open air restaurants next to tables full of young white folks and eavesdrop on your conversation about the volunteer activities of the day. THey have been known to lurk outside the guesthouse where volunteers frequent and try to follow the volunteer to his or her Burmese volunteer assignment. And they mine travelblogs for information, names, and photos. I am certain they are reading this blog within 24 hours of when I post.
You will never be in any danger, but your failure to observe being tailed, imprudent internet postings or overheard unwisely spoken words can have lethal consequences for members of the democracy movement, all of the leadership being in Mae Sot. So remember you came there to help, but first, do no harm.
Some Thai is spoken there, a lot of Karen, and a lot of Burmese. People will speak English to you. Learn numbers in Karen so you can bargain at the market. And bargain gently. Eat rambutan--the best kept secret in delicious fruit unknown to Westerners. Try durian once. Make sure there are a lot of Asian people around watching so as not to deprive them of the huge belly laugh they will have at your expense.
I love Mae Sot, the biggest market for a small town I have ever seen. If you are tired of Asian Nescafe, Dave's restaurant in the heart of downtown has outstanding fresh brewed Hill Tribe coffee and cheese omelettes (something I have not seen since I left on my current travels in November). There is good internet and plenty of ATM's that work for Westerners. The cost of living there is less than half of what it is in Chiang Mai, and 1/4 of Bangkok.
Stay away from the border area after sundown-- it is maybe 8 kilometers west of town so you won't be there by accident. Bicycle is the best way to get around town.
Get your 60 day Thai visa in your home country before you leave, otherwise you only get 30 days if arriving by air and 15 if by land or boat.
(Hey Louise--you should nominate me for local expert for Mae Sot? I know what not to say.)