Shaun, the first thing you need to do is get a free copy of your credit report from at least one of the big three credit reporting agencies. Transunion, Equifax, and Experian. Follow the instructions here.
FTC
When you get it, look it over carefully for any incorrect information. You may find accounts that you still owe money to. You may find incorrect mailing addresses for you. There are many factors that can contribute to a low score and having a credit record that is full of incorrect data and inconsistencies is one of them.
After you've had a chance to make any necessary corrections, and have contacted anyone who is still reporting uncollected debt on you and made arrangements to pay, if there be any, then you are in a position to begin building your credit. Start with a secured credit card such as one of these.
secured cards
They are almost guaranteed to approve you and report to all three credit agencies like regular credit
cards.
In time you will be able to open other credit accounts with department stores, other credit card companies, car loans.
Just use the card/s for essentials and be sure to pay off the balance every month. This is the most sure fire way to build your credit. The more accounts you have and the longer you have kept them current is what builds your credit score points. What they want to see is someone with a lot of accounts who has them all paid off. That means you have a low debt to earnings ratio.
It takes time, but be patient, be diligent, and eventually you will make the credit system work for you. Take it from a guy who filed for bankruptcy once and said "Never again!"
Good luck!