1. Buy from apps
Apps such as Paypal, Square, and Robinhood could help. They are easy to use. You simply sign up for the app's service and start trading. There is usually be a fee, though, usually from 1.5%, for Paypal and Square, but Robinhood doesn't charges a fee, it makes money by routing customer trades through market makers and taking a cut of the spread between bid and ask prices.
Once you bought the crypto currency, Paypal and Robinhood require you to keep your crypto on the platform and sell out in cash, and Sqaure allows you move your Bitcoin to an unaffiliated wallet which you can use to trade with other people.
2. Crypto focused platforms
The largest is Coinbase, it offers dozens of cryptocurrencies and services like lending that are not available from the other apps. It charges a 0.5% spread on transactions, as well as a fee starting at 1.5%, depending on what source you use to buy in.
3. Through a brokerage account
You can buy Grayscale Bitcoin Trust (GBTC) which is a security created by Grayscale Investments that consists entirely of Bitcoin and is structured as a private placement. GBTC is a trust, not an ETF, the difference is GBTC trades at a substantial premium to its net asset value, so as an investor, you have two potential risks: bitcoin's price drop, or the premium shrinks.