How you've paid your bills in the past is usually the best indicator of how you'll pay in the future. Be sure to pay at least the minimum amount required by the date it is due on your account statement or invoice. You can always pay more but you should never pay less than the minimum. Remember – being late on a payment is a negative mark on your credit record, even if you make up the payments later.
Close accounts you never use and try not to draw down too much of your remaining credit lines. Don't apply for too many loans or too many credit cards either. This might be interpreted as a sign that you can easily get in over your head on payments you owe.
You need to have some credit history to have a credit score. Sometimes having a very limited credit record can have a negative effect on a credit score. If you rarely or never borrow money or use a credit card, consider applying for a credit card and using it carefully, paying off the debt each month as required.