You need to show you can use credit / borrow money and pay it back to get a good score.
Some people think that means paying interest -- that is NOT correct.
For example, some think you need to "carry a balance" on your credit cards, and that if you pay the whole bill at once, that's bad.
Not true at all. If you can, perfectly fine to pay your card statement in full.
If you want your score to go up, don't close any credit cards - keep them all open as closing an account reduces your average age of credit account, which is bad.
Also how the algorithms work, if you have multiple cards, have the final monthly balance reported on them be 0 (as in pay off early so nothing charged to you in your bill) on all but one card. So only let one card have a balance that's reported to the agencies.
And keep your credit utilization below 30% - so if you're allowed to access $10,000 of credit, don't go above $3,000 or that looks bad.