Payment history: the biggest factor
This is typically the single largest component of most scoring models. Paying every bill on time, every time, is the highest-leverage habit you can build. Even one 30-day-late payment can meaningfully hurt your score.
Credit utilization
This is how much of your available credit you're using. Keeping utilization under roughly 30%, and ideally under 10%, on each card and overall tends to help. Paying down balances before the statement closing date can lower reported utilization even if you pay in full.
Length of credit history
Older accounts help your average account age. This is one reason closing your oldest credit card, even an unused one, can sometimes hurt your score more than expected.
New credit and credit mix
Applying for several new accounts in a short window can cause a temporary dip. A healthy mix of account types (credit cards, installment loans) can help modestly, but it's a minor factor compared to payment history and utilization.