In order to purchase Bitcoin, you either use a decentralized or a centralized exchange platform to that, secondly i don't know whic country where you reside, so it is another thing to consider because before purchasing of Bitcoin as some exchange might not be available to you in your region base regulations or restrictions especially those that comes from US, or EU countries may face some difficulties access some exchange. Binance, Bybit Kucoin are somethings of the most reliable centralize exchange platforms which you can, whereas for decentralized platform you can bisq.