1. Pay your bills on time. This is the single most important factor to a good credit score. Even if the debt is small it is crucial that you make payments on time.
2. Minimize outstanding debt and avoid overextending yourself by applying for credit needlessly. Use the credit you already have to prove your ability to manage credit responsibility.
3. If you have negative on your credit report (late payments, bankruptcy, or too many inquiries) your best bet is to pay all your bills and wait. Time heals all wounds.