"I am rich!"
You can think of Thì as the equivalent of the copula for adjectives. Eg. It is a cloud: nó là mây vs it is beautiful/cheap/old: nó thì đẹp/rẻ/cũ. Mây is a noun, a thing. Beautiful/cheap/old are all adjectives aka descriptions. If the object of the sentence is an adjective, use thì in the function of "is" (more or less).
I am rich! Possible translation: Tôi giàu! Tôi giàu có! Tôi giàu rồi! When you talk to your friends (If you don't want to use word "mày-tao (you-me)") Mình giàu rồi! Mình giàu có! Mình giàu có rồi! When you talk to your friends (in close way. Use "mày-tao" when you're talking to your partner who is younger or have same age with you) Tao giàu! Tao giàu rồi! Tao giàu có rồi! -From native Vietnamese