If you're an authorized user on your dad's account, no issue with you being under 18. Your dad would be responsible for the entire debt on your card (and his card, too). Some cards have a major credit card logo along with the words "Debit Card". If yours does, you can use it as a debit or credit card. Here's how it works:
The following is presuming your debit card has a major credit card logo (Visa, MC, etc). If it is just a debit card without the logo, you cannot make online purchases. Also, this is presuming your debit card is linked to some account with funds on deposit (usually checking).
This is how mine works:
1. Credit cards: Your purchase is recorded almost immediately (within seconds), but you don't have to pay for your purchase until you receive your statement. Your checking account remains untouched.
2. Debit cards: Your purchase is recorded almost immediately (within seconds) and you pay for it directly from your checking account. The funds must be in your checking account to cover the cost of your purchase.
3. Debit card without a credit card account. Your purchase will be converted to a debit purchase and processed as #2 above. You don't need a credit card account to process your payment as a credit card transaction.
4. Debit card with a credit card account. Your purchase will remain as a credit card purchase and be processed as #1 above.