Before giving more information, I think the first thing to know about the countries which get extra privilege of getting a Visa-On-Arrival. Currently, The Kingdom of Thailand has given special Visa-On-Arrival status to 28 countries around the world, which are as follows
The kingdom of Bhutan
The republic of Bulgaria
The people's republic of China
The republic of Cyprus
Federal democratic republic of Ethiopia
The republic of Fiji
The republic of Hungary
The republic of India
The republic of Kazakhstan
The principality of Liechtenstein
The republic of Malta
The united Mexican states
The republic of Nauru
The sultanate of Oman
Papua new guinea
The republic of Poland
The Russian federation
The kingdom of Saudi Arabia
The Slovak republic
The republic of Slovenia
Taiwan - China
The republic of Uzbekistan
The republic of Vanuatu
These countries can avail visa at any international airport or Immigration Checkpoints in Thailand. These country people get 15 days tourist visa from Thai Immigration for ฿2000 & sometimes you can get visa for free depends on the month you are travelling.
The travelers who are from there countries should meet all the criteria required to get a Thai visa. The requirements is as follows
Visit is for tourism purpose only
Passport should be valid for at least 30 days.
A valid address of stay in Thailand (hotel booking etc)
A confirmed return air-ticket within 15 days period (Open tickets are not accepted).
At least ฿10,000 per person and ฿20,000 per family cash should be shown (This excludes visa charges)
Tips to make it smooth
The visa on Visa-On-Arrival at airports take time and if you want to make it little quicker, then these tips might help you.
Submit your visa form online and print it well before going to Thailand (Official Link: CLICK HERE)
Keep all the required document in a file and hold it in your hand
Carry a small pouch with your cash money and hold ฿2000 for visa ready
Carry a pen
Smile at the immigration officer
However, if you are not eligible to enter Thailand under the VISA ON ARRIVAL and VISA EXEMPTION RULE then you can obtain visas with the Royal Thai Embassy prior to your visit.