There are more than 100 crypto exchanges on the market. Go with a reputable exchange like Binance, Coinbase, KuCoin, BlockFi, eToro, or Crypto.com.
Do your homework. What are the service fees, what devices are supported, how secure is the site?
Next open an account. Reputable exchanges will require you to submit official data and documents. It's about "know your customer." This is a federal regulation to make sure the exchanges are legit.
You don't have to invest in Bitcoin. You can explore other cryptos like Ethereum, Solana, Dogecoin, and many others. But do your homework. It's a high risk investment.
Most exchanges will allow you to make your purchase with a debit or credit card in some cases. Once you make the purchase, check the confirmation and set up a crypto wallet to safely store your investment.