There are a lot of things that go into your credit score. Having a large credit line that is not being utilized is actually a good thing. However, if you have a credit line with the majority of it being used then that is bad. It means you have a lot of debt out there.
If you want to lower your debt, payoff some of your debt, do not necessarily close accounts once they are paid off (credit cards) as this will lower your available credit, and do not default on any accounts. Building up your credit takes time.
When I first met my husband I didn't have much credit just because I really hadn't taken out any loans and didn't have credit cards. We got me a credit card that we paid on every month. When we went to buy new furniture, I got a small loan from my credit union (they have better rates), and then paid it off ahead of schedule. These things quickly built up my credit score.
If you have poor credit, it will take time to build it back up. That is because the credit bureaus keep certain information in there for a set period of time such as late payments or defaults. So keep up with your payments and pay down your debts. Also, be sure to check your credit rating every year (as it is free to do so) to make sure everything looks good.