Payment history is usually the single largest factor: consistently paying credit card bills and existing EMIs on time, every time, matters more than almost anything else. Even one or two missed payments can noticeably drag down a score that took years to build.
Credit utilisation — how much of your available credit card limit you're actually using — also matters considerably. Keeping utilisation well below your total limit (commonly suggested under 30%) signals healthier credit behaviour than regularly maxing out cards even if you pay them off in full each month.
Other factors worth managing before a loan application: avoid applying for multiple new credit lines in a short window, since each hard inquiry can temporarily ding your score; don't close old credit cards right before applying, since a longer credit history generally helps; and check your credit report for errors, since incorrect entries (a loan that isn't yours, an outdated missed payment that's since been resolved) are more common than people expect and can be disputed and corrected.