I think you have already been given answers to your question already, but I will still share my ideas with you. However there are different ways to purchase Bitcoin, one is through an exchange like binance, kucoin, bybit or Bitget, you can chose any of these platforms I suggested for you, through their P2P function you can buy bitcoin with as low as $5. But on the other hand, buying Bitcoin is not the problem, the important thing is how can you secure your Bitcoin after purchasing them? Another thing you should know is that keeping your Bitcoin in CEX is not safe because you don't control your assets.