Pack light! You don’t need much. I have dozens of friends who have lived out of one backpack for 6 months, and keep in mind that I moved to Asia for a year and a half trip with only two bags. Keep in mind that Thailand is a tropical country where you rarely need any cold-weather clothing or jackets. Laundry is very cheap and you can purchase anything you need over here, so pack less than you think you will need.
If you want to visit Thai temples (wats) you should bring one pair of lightweight long pants (and women need to cover their shoulders and arms).
There are 7/11’s on every street corner where you can get snacks, basic toiletries and household items. Tesco Lotus is a good chain to find cheap food and household items, and there are plenty of inexpensive, fairly good-quality clothes to be found among the street vendors at Khao San Road, Chatuchak weekend market, and the innumerable street markets that crop up all over Bangkok.
Don’t forget to barter! It’s a way of life in Thailand. This won’t work for food most of the time, or in nicer hotels, department stores, etc. But if you’re buying goods on the street, ask their price and then start your counter offer at half what they quoted you.