Credit Score Management

What I want to know is how to clean up my credit report of items that have already been placed there.

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I received a letter in the mail from Juniper (iTunes Rewards) stating that they have closed my credit card account due to inactivity. I opened the account about 18 months ago and have never really used it. Can they close my account just like that without any warning? How should I handle the situation? (Is there anything I can even do?)
Also, I know that closing credit card accounts lowers your credit score. Does anyone know how much mine will be affected?

I have alot of derogatory items on my credit that I really need to take care of. I have opened some new credit card accounts since I have never had credit cards before. What is the best way to improve my score so that I am not such a high risk?