How do prepaid credit cards work to shop online?

I’m 14 and I’m wondering how prepaid cards work? Can I just go to CVS buy a $10 card with cash and then use it online or will they send me some additional bill. Do I have to activate the card myself or is it automatically activated when I buy it? I don’t know much about credit cards.
Is it like a Gift Card but you can use this card in any store compared to a gift card?

they sell "giftcards" almost everywhere. they have visa, american express etc. make sure to buy a prepaid GIFTCARD because they dont have send you bills. they sell them in 10,25,50+ i think. but they charge a 5 dollar fee. so if you want a 25 giftcard then you’d probably end up paying 30 bucks. if you only want to order from one big name store like target or american eagle then you should go to their store and buy a giftcard because they dont charge you extra for their own giftcards. i hope you understand this:) oh and a visa giftcard is accepted where ever visa credit cards are accepted!