Next to payment history and your credit utilization ratio, "length of credit history" is the most important factor in determining your FICO credit scores. In fact, it accounts for approximately 15% of your overall credit score, so it's definitely something you can't ignore. So what does this have to do with choosing your first credit card? A lot! You see, your length of credit history is primarily determined by the amount of time that's passed since your first credit card was opened, which means you want to keep your very first credit card open for as long as possible. That may sound easy in theory, but it's actually quite difficult for many credit card users.