You can do this yourself by going to the credit reporting agencies' web sites i.e., Transunion, Experian, and Equifax, and disputing items on your credit report. If you don't know your credit score, you can go to Creditkarma.com and get it for free. You can also get a copy of your credit report for free once per year per agency at annualcreditreport.com but it does not have your score.
I would not recommend paying someone to work on your credit for you unless you are very sure they are reputable and can help. There are a number of non-profit agencies that can assist. Be careful of scammers.
By the way, I am a property manager and we do require a minimum credit score under most circumstances. We will occasionally look at the reasons for the bad credit such as illness or job loss but the market is very competitive right now. All landlords will be looking to lower their risk of not getting the rent and a low credit score definitely puts you at a disadvantage, as you clearly know.