Credit is not all that hard to understand, but it can be difficult fix problems on your credit report.

Scores are based on the number of open credit (trade) lines you have open. Lots of open lines -> lower score. You need about 3-4 open trade lines, e.g. credit cards, charge account, installment loans.
Then, you MUST pay these lines as agreed. Late payment are very bad because they remain on your report lowering your score for 24 months. They continue to stay on your report for longer but with less effect. Recent late payments are the most damaging. So, pay on time.

Housing-related credit lines can damage your ability to get a mortgage loan. So, back rent liens, utility collections, and the like can destroy your ability to get a housing loan, even though your score isn't that bad.

Collections lower your score even more. As an account ages from 30 days late to 60 days to 90 days, the damage grows. When it finally is sent for collection, it's even worse. Try to avoid having any accounts sent to collection. It is better to have a $10/month payment plan than an account sent to collection.

Charge-offs are the pits. Once an account cannot be collected, it may be written off completely by the creditor. If the charge-off is more than 3 years old, it has done its damage and is done (until another collection agency takes it over). Much as I personally disagree with the practice, many collection agencies trade old accounts with each other. When they receive an old collection account, they immediately go report as a new collection even though it's years old. I don't think this is fair. After 5 years, can it really be a recent debt?

Foreclosure, repos, and bankruptcy are at the bottom of the pit. They are the most damaging and long-lasting items.

Why tell you all this? Because credit repair is about dealing with the problems and correcting them. Occasionally, errors are made in reporting - first remove these by bringing them up to the credit bureaus
Second, make sure you pay all your open account on time. Lates can override your efforts.
Third, work through the delinquent and collection accounts and unless they are ancient, make a plan to pay them off. If they're not yours, dispute them; if they're yours, pay them.
Fourth, don't open any new accounts.Recent account have no history of on-time payments.
Fifth, don't close accounts with long histories behind them. If you have 5 credit cards and want to close 2 of them (you don't have to), pick the 2 that have the shortest history, not the very first one you got. Paying on time on a long-standing account helps you a lot. If you close the account, only the bad accounts will be left.
Sixth, keep your account alive by making periodic nominal purchases every few months.

If you need the names of some people I have used in the past, please ask, but you can do everything that needs to be done without paying someone else.