1. Check CoinMarketCap to see where you can buy THETA and with which currencies
For each cryptocurrency, CoinMarketCap provides a list of purchasing options (also known as market pairs). Go to CoinMarketCap and search for THETA. Tap on the button labeled “Market” near the price chart. In this view, you will see a complete list of places you can purchase THETA as well as the currencies you can use to obtain it. Under “Pairs” you'll see the shorthand for THETA, THETA, plus a second currency. The second currency is what you can use to purchase THETA. If you'd like to purchase THETA with the U.S. Dollar, look for THETA/USD.
2. Pick a platform to make your purchase
Different platforms have different levels of security, reliability, and liquidity. Before creating an account, do your research.
3. Make the purchase on your chosen platform
Every platform has a different way of doing things. Some platforms are very easy to use, others not so much.
Generally speaking, purchasing crypto with a fiat currency like the U.S. Dollar will be easier than purchasing it with another crypto.
If you do have to purchase THETA with another crypto, you’ll need to first create a crypto wallet that supports THETA, then you’ll buy the first currency and use it to buy THETA on the platform you chose.
If you get stuck, most platforms provide guides. But if they don’t, there is a vibrant community of crypto enthusiasts that have likely posted guides on Youtube, Twitter, and elsewhere.