At the airport (luggage pickup) you will see a TrueMove-H stand (usually unmanned) with free SIM cards for travellers. They have a very small deposit on them also, but can be topped up at any 7-11 (millions of them!). These are also preset with important numbers such as tourist police and EMS services and immigration etc. They are also set to receive English script text. Thai SIM cards allow for English and Thai. Non Thai phones generally do not decode Thai so you get texts with little boxes instead of characters. They have a limited data account too, but will need to be topped up to use this. Activation instructions are in English.
Three main providers (DTAC/AIS/True) all have pretty much full coverage. 3G is limited by all of them to towns, flipping to Edge in the suburbs. There is no 4G everywhere in Thailand you are likely to travel to (maybe not in the middle of the rainforest!)
Rest assured which ever SIM you chose expect plenty of adverts by text!