Five things feed the score. They don't carry equal weight, and only one of them changes fast.
Payment history is the heaviest piece. A single payment reported 30 days late can cost more points than a maxed-out card, and it stays on the report for seven years. The fix is boring and it works: autopay the minimum on every account, then pay the real amount manually. A distracted month then costs you a little interest instead of a mark that follows you around until 2033.
Utilization is second and it's the fastest lever you have. It's your reported balance divided by your limit, measured card by card and across everything. Scoring models generally like it under 30%, and the strongest scores tend to sit under 10%. The part that trips people up is which balance gets reported. It's the one on your statement, not the one left after you pay. Pay the card down before the statement closes and it reports a small number even in a month you ran $4,000 through it.
Age of accounts, credit mix, and new inquiries split what's left. This is why closing the card you've had since college can drop your score. You lose the limit, which pushes utilization up on everything else, and eventually you lose the history that card was carrying. Keep it open, put one small subscription on it, autopay it, and forget about it.
Checking your own score is a soft pull and does nothing at all. Applying for credit is a hard pull and costs a few points for a few months. Shopping several mortgage or auto lenders inside a short window usually gets bundled into a single inquiry, so compare rates without panicking about it.
If you're running any strategy that keeps a balance moving through a line of credit, the utilization piece behaves differently than you'd expect, and I covered that over at what paycheck parking does to your credit score.