Utilization and paying in full
The fastest mistake I made was thinking carrying a balance builds credit. It does not. It just costs you interest.
What utilization is
Utilization is your balance divided by your limit. If you have a $1,000 limit and a $300 balance, that is 30 percent. Lower is better; under 30 percent is a common guideline, under 10 percent is better. It is one of the biggest score factors and it updates monthly.
Pay the statement in full
Paying the full statement balance every month means you owe no interest and still build a perfect payment record. Carrying a balance does not help your score, it just adds interest. Autopay set to the full statement balance protects you from a slip.
Higher limits help
A higher limit lowers your utilization at the same spend, which is part of why a credit-limit increase can help your score, as long as you do not spend more to match it.