Citizens of most countries can enter Thailand without a visa and stay for up to 30 days, but if you plan to be in Thailand for more than 30 days, then get a travel visa for Thailand -- either a "tourist visa" if you are here just for tourism, or a "nonimmigrant" visa if here for business, both good for 90 days. A "visa" is not a credit card, it is a stamp put in your passport by a Thai embassy or consulate in your country. You can usually mail your passport in rather than needing to travel and visit in person. I strongly suggest registered mail. For a nonimmigrant business visa, talk of email the consulate for particulars.

Nationals from 56 countries can enter Thailand for 30 days without a visa, whereby you can get the visa stamped into your passport from an immigration officer at the airport. This is called "visa upon arrival". Nationals of another 76 countries, most of which don't have a Thai consulate, can get a 15-day visa stamp upon arrival. Nationals from Sweden and Korea can get a 90 day visa upon arrival. However, these are supposed to be used for tourists, not people intending to work and/or live in Thailand.

Nationals from countries near Thailand from which many impoverished illegal workers come often have problems (e.g., Bangladesh, India). Immigration officers have the power to deny a visa, including a visa on arrival at the airport (whereby you could be put on the next plane going back with available seating), so if there's any doubt, get a visa before you come, and check the minimum amount of currency you must have on you (in one form or another) which varies by visa category.

If you plan to be in Thailand longer, then you should bring your passport to the Thai embassy or a Thai consulate nearest you in the country where you are currently residing, and apply for whichever visa is appropriate:

Visa on arrival (15 days)

A visa on arrival or transit visa is issued upon arrival in Thailand for those who arrive without a visa, and is similar to a tourist visa except that you get only 30 days, and it requires a ticket for continuing your travels back out of Thailand.

Tourist visa (90 days)

No special paperwork is needed for westerners from rich countries and those from rich Asian countries.

Non-immigrant visa (90 days, renewable to 1 year)

If you plan to do business here, or reside with a spouse, or retire.