I really am a noob (A thread about purchasing online)

[SERIOUSLY THOUGH]

Well first of all, it'd be useful to know your country because different countries have different security measures.
Main thing to do to buy with a debit card is
1 - Choose your item
2 - Go to checkout
3 - Select "debit card"
4 - Put in all your details
5 - Normally at this point, you'll be forwarded to your bank's page for more security questions
6 - Receive confirmation of purchase.

Do be careful though on which websites you use your debit card details. I only ever use mine for Amazon and big retailers that I trust. Remember, the numbers you put in can be saved by the other person and used to steal your money.
If you have a credit card, use that as you can then contact your card supplier and say "wut, I didn't buy this" and often they can give you your money back.
If you don't, ask if you can use your parent's credit card to buy things. If this is your first debit card, don't suddenly think "heyyyyyy I can buy things without my parents knowing" because that'll most probably lead to a LOT of spending