As you know we do 'Wai' by put both of your hands on your chest and bend your head a little bit to hands.
'Wai' is for
1) Greeting
2) Thank
3) Good Bye
4) Apologize
5) Praying to the Buddha
6) Begging
But!!! Do you know ? We don't 'Wai' to everyone. Thai people pay respect to someone who is older as senior and we respect them as boss in a service business. So, please don't 'Wai' to anyone who is younger than you. Younger people won't feel comfortable to get your 'Wai' first. But if they do to you, you'll have to accept them back by do it the same after that.
Don't have to 'Wai' everyone who you don't know. For example; if you buy something in the shop, you don't have to 'Wai' the owner because you're the customer unless they invite you to stay over or join food with them. 'Wai' must be feeling or meaning to that person.
But whether you do right or wrong, Thai people are still happy to see you do that because we like what you try to do as our custom.