Depends where you live, but Coinbase is a popular exchange that lets you buy crypto with fiat currency. Create an account, and then you can buy crypto (e.g. eth) with either a credit or debit card. I would recommend debit, as it seems that Coinbase recently changed there credit card transactions to appear as cash advances on your credit card bill.
From there, if you wish to just hold onto it with the hopes that the value will increase, you can set up an eth wallet and transfer your eth from coinbase to your wallet.
Please note that there is no guarantee you will make any money, and it is entirely possible for you to lose some or all of the money you put into crypto. Do not invest anything you're not prepared to lose.