You didn't say what you want to trade so I'll assume it's Bitcoin vs USDT or fiat.
Go to a website like https://www.worldcoinindex.com/coin/bitcoin or https://coinmarketcap.com/currencies/bitcoin/markets/ and get one from the top, by liquidity or 24h volume.
Don't forget to read the ToS, to avoid surprises; in many cases you'll probably have to do KYC.