At 18-25, you're still learning. Here are the specific traps that destroy credit scores and cost thousands in extra interest:
Closing old credit cards after paying them off: Many people think closing accounts is smart. It's actually backwards. Closed accounts stop aging, which reduces your average account age. They also remove available credit, which increases your utilization ratio on remaining cards. If you had a $2,000 limit across two cards and close one, your utilization spikes instantly. Instead, keep old accounts open and use them occasionally (small charge, pay it off).
Co-signing loans for friends or family: When you co-sign, you're legally responsible if they don't pay. Their debt appears on your credit report. If they miss a payment, your score tanks. If they default, creditors can come after your wages. Decline politely. Your credit is worth more than this relationship.
Ignoring collection accounts: A bill sent to collections doesn't disappear if you ignore it. In fact, waiting makes it worse. The statute of limitations (how long they can sue you) is typically 3-6 years depending on your state. If you have collection accounts, consider paying them. Once paid, many credit bureaus will remove them or mark them "paid collection," which is better than unpaid.
Maxing out credit limits: Using 100% of available credit signals desperation to lenders and destroys your score. The number-two reason young adults fail to build credit is carrying 80%+ utilization. Keep it under 30%, ideally under 10%.
Making multiple credit applications in short periods: Each application (hard inquiry) drops your score 5-10 points and stays for 12 months. If you need credit, research before applying. Apply strategically. Space applications 6 months apart if possible.
Using rent-to-own or payday loans: These predatory options charge 300%+ APR. A $500 payday loan costs $650+ to repay in two weeks. They don't build credit and trap you in debt cycles. Avoid entirely.