Traveling to Thailand: what do i need besides a passport?

Traveling to Thailand: what do i need besides a passport? - 1

A visa. Check to see if your country qualifies for visa on arrival. Money, bring enough cash for emergency, a few traveler's checks, and best an ATM card. Check with you bank to make sure it will work in Thailand.
Search out various travel forums etc to get latest updates on what else you might be advised to know. A bit of research now can prevent major problems and headaches later.

Pretty much everything you need if you were going to another country. Treat Thailand just like any other country, so bring what you would bring normally. Obviously a Visa, money, a backpack, mosquito repellent, etc. Don't bring too much though, because everything you need will be on sale here at a cheap price.

And also, you may need some sort of translating tool to help you. A reliable phone with Google translate, a Thai dictionary of some sort may help.

You didn't state where you're from or where in Thailand you're going, but in general I think the important things are:

Suncream/aftersun/hat - It's hot all year round in Thailand and some people, especially those who are from northern climates, might find it scorching. If you get sweaty easily then it might be a good idea to prepare some extra t-shirts so you can change during a hot day.

Basic knowledge of Thai culture - lots of people tend to ignore this but I have seen so many tourists ruin their own holiday by doing something that got them ended up at a police station, kicked from places, and injured. Last year a group of tourists decided it was a great idea to climb on a buddha statue and for some 'cool' photos. That upset a lot of Thai people so a fight broke out and everyone ended spending their night in a cell. I don't think that's how anyone want to spend their holiday.