Just turn up in Thailand with your passport and a ticket out of the country. Currently good for 60 days, extendable for another 30 by going to a Thai immigration office in Thailand a week before the 60 is up.
There are quite a few immigration offices. Google maps for locations.
What is your nationality? Currently the citizens of 93 countries are visa exempt (that means you just turn up at the airport in Thailand and Thai immigration will just stamp your passport). You get 60 days, which can be extended a further 30 days by going to an immigration office in Thailand 3 or 4 days before the 60 days is up at a cost of ฿1900. Go to the immigration office before this to find out about passport-size photos and the TM30 form you get from your accommodation.
Small backpack 25L and a waist pack. Buy slide sandals or flip flops from your own country. Big plastic bag for the sandals. Wear breathable training/running shoes and bring three pairs breathable thin socks for travel. 2 t-shirts. 2 lightweight shorts. Leave room in backpack for 100฿ elephant long pants for temple visits - buy those here in TH.
Passport in clear sandwich plastic bag. Debit card (the charge is 220฿ per transaction - use the yellow kungsri ATMs because you can get 30,000฿ per transaction, and remember to hit the button bottom left “NO conversion”.
Keep total below 7kg. Backpack and waistpack. Then you can fly around Thailand with no extra baggage charges.
Nokair.com is one of the cheap airlines.
Bring an extra debit/credit card or two.
Bring an extra bin bag to put your stuff in in case of rain.
Agoda.com and booking.com to book hotels/hostels in advance.
Grab and Bolt apps for taxis.
Airport train from Suvarnabhumi Airport basement (Bangkok) into city, or direct bus to Pattaya/Hua Hin from the airport.