After spending a year in Thailand, many options are offered to you when you want to travel in this country:
-the bus: https://12go.asia/en. It is not expensiv but it can be very long, try to choose a good spot…and the bus have AC!
-the local bus in Bangkok, who are inexpensiv, the yellow buses have AC and the other one doesn’t!!! The cost is around 8 and 24 baths depending on where you are going (less than $1) https://www.transitbangkok.com/bangkok_buses.html
-the train, with two or three classes. The thaï uses the second or the third class who is not expensiv and the travelers who likes not to pay a lot for travelling (the windows are open wide so this is how you get AC) http://www.railway.co.th/main/index_en.html
-the plane: Nok air https://www.nokair.com
or AirAsia.com https://www.airasia.com
You can pay directly online or you can choose to pay directly at the 7-11, very convenient and cheap (but most of the departure are in Dom Mueng, so one hour from Suvarnabhumi, they have a free shuttle between these two aeroport if you show them your plane ticket. What is great with Nokair, they manage the plane, the bus and the boat if you want to go the islands (just follow their signs!!!)
-Taxi, in Bangkok, always asked that the drivers is putting the meter on….or they will try to charge you too much…
-Grab, which is not the cheapest solution if you compare with a taxi with his meter in Bangkok (but asked them to put it on!!!They will try to charge you more)
-Song Thai, in Pattaya for example, it is only 10 baths wherever you go…great deal!!!And even sometimes, you can ask them to drop you where you want and you can bargain the price!!! I didn’t find actually a map of their road, so you kind have to ask them!!!
-Bike, a good solution if you are near the beach (not in Bangkok and Chiang Mai, it is too polluted)