Review your credit report carefully and mark every suspected error. If your credit report includes incorrect information, contact the credit reporting agency immediately. They will review your report, investigate your claim, and make the correction if one is called for.
Even if the negative items are true, you can still make things better. If the entry refers to an unpaid debt, contact the creditor to negotiate payment options. If there has been a dispute, you can ask the credit reporting agency to add a note to your credit report explaining your side of the story.
The good news is that, as you improve your credit habits, most of the negative entries may eventually fall off your report over time. This is more the reason to start as soon as possible managing your credit effectively.