Two levers move a credit score fastest. The first is credit utilization — the share of your available credit you are using. Paying balances down so you are using less than about 30% (and ideally under 10%) of your limits can lift your score within a statement cycle or two. Asking for a higher limit, or spreading balances across cards, can help the same way.
The second is payment history, the largest factor in most scoring models. Bring any past-due accounts current and never miss a due date going forward — autopay for at least the minimum is the simplest safeguard. Beyond those, avoid opening several new accounts at once, keep older accounts open to preserve your credit age, and check your reports for errors you can dispute. There is no instant fix, but utilization and on-time payments deliver the biggest, quickest gains.