Credit is like a muscle, if you use it properly it will get stronger. If you damage it, it hurts and it will take time, medicine and care to restore.
Start by creating a budget that includes all of your bills, savings and advanced planning for items like auto repairs, gifts, vacations and other expenses you can anticipate that may not be monthly. It should include some savings for an emergency fund for the unexpected and a down payment for your house.
Once you've done this, review your credit report for errors. Dispute anything you know is wrong in writing to the specific reporting agency that reports it. Include proof for what you are disputing.
Pay down your outstanding balances to 1/3 of their limits or lower. The more available credit you have, and aren't using, the better your scores.
You can talk to your local bank or credit union about getting a secured credit card. If you deposit $500.00 you will have a $500 limit (or there about). Use the card for items you are buying anyway like gas, groceries etc and pay it off in full every month.
This should get you started to better credit. If you have specific issues you may want to talk to a lender who has seen your report and knows something about credit repair.