In order to buy the Theta token, you must first exchange U.S. dollars to some form of cryptocurrency.
There are many ways to do that, but for simplistic purposes, we will deal with Coinbase a popular exchange in the United States. I have found that LTC litecoin swaps at a favorable rate in Simpleswap.
Use this link and we both get $10.00
The next step is to set-up your Theta wallet.
I encourage you to take the time to explore this site.