How to buy Polkadot dot?

Could you please explain, in detail, the process of purchasing Polkadot dot? I'm interested in understanding the steps involved, from setting up an account to actually making the purchase. Also, are there any specific requirements or prerequisites that I need to fulfill before I can start buying Polkadot dot? Additionally, could you provide some insights into the best platforms or exchanges to use for this purpose? I'm looking for a reliable and secure option to make my purchase. Thank you for your assistance.