Before you visit any country, you should do a little research to find out about the culture and see if there is anything you do that might offend or be illegal. Here are a few you should be aware of.
1. Don't point your feet at Buddha or anyone for that matter. Feet are considered the lowest, dirtiest party of the body so to point them is considered especially rude and disrespectful. Inversely, don't touch people on the head, the highest and holiest part of the body.
2. Don't touch monks, especially if you're a woman. If you want to give a monk something, give it to a male companion first and get him to pass it on. Also, always let them go before you and offer them space to stand on the ferry.
3. Don't be naked. This seems like a no brainer but I still see naked peeps every time I go to the beach. Thais, despite the stereotype as a highly sexualized nation, are generally very conservative about their bodies. Public nakedness is a big no no. So when you go to the gym, don't get changed in front of everyone. There are little change rooms you can go into. And for heavens sake! Don't sun bake topless at the beach. I know you don't want tan lines but get over it. Tanning is bad for you anyway. I am talking about guys here too though! I see men (hairy, sweaty, fat men at that) walking around with no shirt on. This is also offensive.
4. Don't point at pictures of the King or talk about the royal family in public. Everyone, and I mean everyone, in Thailand loves the King. Pointing at him is considered rude and talking about the family is actually illegal. If you must point out a picture of the King (they are everywhere) use an open palmed gesture.
5. Don't bring in anything related to "The King and I". Strange I know and I have no idea why you'd bring anything unless you're a die hard fan that can't live without it, but anything to do with the King and I is illegal here. It portrays Rama IV or Phrabat Somdet Phra Poramen Maha Mongkut, Phra Chom Klao Chaoyouhua and was very shocking when it first came out in Thailand because of it's inaccurate portrayal of the King and his relationship with a foreigner.
6. Don't litter, spit or drop cigarette butts, even if you see Thai people doing it. It's very illegal and you'll get a nasty fine if a policeman sees you. Plus it's dirty anyway and gives foreigners a bad name.
7. Don't dress like a skanky hoochie Mamma or a complete Bogan at temples. You MUST cover up at temples. So pants and shirts that cover your shoulders are required. Women AND men!!
8. Don't do drugs mmmkay. They are really, really, REALLY illegal here.
And just generally be polite and friendly not a stupid, loud, ignorant tourist. There are far too many here already!