Start off in Kao san road in BKK there are backpackers from all over the world doing the same as you and hotels are cheap enough.

All your internal travel can be arranged there everywhere.

If you use the Buses and trains in Thailand they are pretty cheap and you will save on accommodation if you travel at night!

Food is really cheap if you stay away from western style restaurants.

I think most backpacker use "Lonely Planet" to get general tips for where is best to go.

Samui may have the best weather at that time of year for beaches,and Koh Phangan is a must for the Full moon/Half moon/black Moon party.

My backpacking days are long gone but if I was backpacking Thailand now I would not come home for a year!

Look up Sawadee.com - Thailand tourism portal - Hotel and Resort reservations it is a pretty good site that will give you maps and info on road/rail/ferry/air travel,best times to visit and a bit of history of each place ect. Most of the Hotels might be out of your reach with your budget but its fun to look at what you can get for not much money.

Den