For those who don't have a credit card, or who are underage and cannot legally obtain a credit card, there are a lot of prepaid credit cards out there that you can load with money to use online and elsewhere (like a gift card, only a Visa or Mastercard that you can use just like a credit card almost anywhere).

For instance, Walmart sells a Visa 'moneycard' for $3.00 that allows you to put anywhere from $5 to $500 on it. Then you can use the card just like a credit card until the money you loaded onto it is used up. Walgreens also sells prepaid credit cards, as do many grocery stores and other retailers. I myself can vouch for the Walmart and Walgreens versions of pre-paid cards, I have used both myself

Hope this helps!