You’d need to open an account on an exchange. Some of the global exchanges that people round the world can register at and use to fund their membership are Bittrex (bittrex.com), Binance (binance.com) or OKX (okx.com).
On creating an account, you’ll be given your own unique Crypto wallet. Once you’ve uploaded the necessary documents to have your account verified, you can fund your account with your local currency and with it, purchase USD Tether ERC20 or USD Tether TRC20.
It’s a relatively simple and straightforward process.