I am going to list some simple rules to improve your credit line which I learned by reading some experts and then followed them in my life:

1. If you already have credit lines open then don’t overuse them and always pay on time.
2. If you can’t pay in full then at least pay minimum required on time always.
3. Never utilize more than 30% of your credit limit on each card. If you do so it impacts negatively to your credit score.
4. Do not apply for more than 2 credit cards in a year. Every hard credit check impacts negatively.
5. Do not close older credit cards, even if it doesn’t give much benefit. Length of credit history matters.
6. If you have bad credit or no credit don’t worry. It takes some time but easy to repair or improve your credit score.
7. Open a secured card by depositing some money and use that for some period of time wisely (the steps 1–3 above) to improve your credit score in less than a year.
8. Apply for Discover Credit card as they usually approve for credit low score as well and use it wisely (the steps 1–3 above) for improving or rebuilding credit score.
9. Do not apply for retail store credit cards which are of no much use in building credit but hard hit for credit check may negatively impact.