Here is what you will need to do:
Choose a regulated exchange or broker. Find one that operates in your country and is licensed there like Coinbase, Kraken, Binance or Bitstamp. Verify its availability in your area.
Open an account. You will have to do Know Your Customer (KYC) verification. This will involve providing an ID as well as proof of residence in some cases.
Deposit funds. It can be via a bank transfer, debit card, or P2P trading. The least expensive method is typically a bank transfer. Cards tend to have higher fees.
Place the order. A market order buys at the current market price immediately. A limit order buys only at the price set by you. Coins can be bought in fractional amounts, thus $20 is sufficient to start.
Protect it. Set up two-factor authentication and for large amounts move coins to your own wallet. Hardware wallets are the safest ones. You should store the recovery phrase offline.
Caveats
Coin prices fluctuate greatly, thus you should not risk anything you cannot afford to lose.
Stay away from "guaranteed return" programs, giveaways and unsolicited investments advice – they are very common scams.
Make sure to check the local laws and taxes because they vary by country.