Hi! Just 2 or 3 years ago, my credit scores were all about 520, if I remember correctly. Now, all of my scores are 700+.
Just from my experience, here's what I did.
First thing's first, pay your bills. That's obvious. If you can't afford something, don't even consider it.
The first thing I did to start increasing my score was I went to Aaron's and got a laptop. You overpay for anything you get like that, but my score went up 30 points once I paid off the computer.
Then, I got credit cards. 2 of them. The trick I learned is to NOT USE THEM. lol Or at least use them minimally. What I do is I have one of my small monthly payments set up on both cards. One card is connected to my Netflix bill, and the other is connected to my Six Flags membership bill. Less than $20 on each. I have them both set up on autopay. After that, I just completely ignore them. I don't use them for ANYTHING else. And my credit score has just been going up, up, up. One of them is at 720. The trick to that is you don't want to put a lot on your credit card. That actually lowers your credit.
That's all I've done, besides signing up for Collection Shield 360. (Google it, I'm not sure if I can post outside links here.) They help get rid of negative accounts on your credit for free. They got rid of medical bills on my report.