keep below a 30% utilization of your credit limit, do not use more it harms your score. Typically you want to have more cards as it shows you are a responsible borrower, this does not mean open as many card as you can just have a manageble amount of cards. Always pay you bill on time.
Score improves for a number of reasons depending on which credit bureau it is you’re getting your score for. Generally you want the amount on your card to be as low as possible when it gets reported to the bureau each month. That day varies from bureau to bureau and between credit companies. So just try to pay off immediately before it’s even posted to a monthly statement if you use it like a debit card.
Other reasons include hard inquiries, average length of account age, history of missed payments, and more I’m forgetting. Hard inquiries occur when you apply for a loan or a credit card and they do a hard check. These are inevitable but it’s best to spread them out by 6 months to a year. Account age you wanna keep up by continuing to renew and use your oldest credit card. Missed payments are self explanatory. I recommend getting credit karma, free and shows you info about your credit