Pre paid Visas work just like regular credit cards. The merchant on the other end wouldn't know the difference. Say for instance if you went on amazon.com and you select what you wanted to buy, you go to your shopping cart and follow the steps asked. Once you get to payment section just enter card info, verify, submit and then you'll get a confirmation in your email of your purchase with follow up emails confirming shipping with tracking numbers and estimated time of arrival. Most online sites work the same way.
As for PayPal, you may want to check into minimum age requirements and its been a while but if I'm not mistaken, pay pal requires you to have a bank acct to attach to it. I think there is a non bank option but I don't remember how it works and if paypal still allows it. You may want to go to their website and check out their FAQ.
Also if in doubt, ask a trusted adult for help and advice so that your age and inexperience aren't taken advantage of.
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