Look at Air Asia or Nok Air for cheap flights to a lot of domestic destinations in the country.
Try seat61 for train information.
Try Travelfish for information about destinations.
I did 10 days in Japan with only a 16 litre bag, so in Thailand you could easily take a 19 litre pack and carry all you need.
Just wash clothes in the sink a few times, or laundry services are cheap. Or just buy as you need.
For example, wear shoes on the plane over, and buy flip-flops here (between 2-6 pounds for a cheap pair).

I live here, so just ask me whatever and I'll try to help.