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.
Experian is rolling out a new thing called Experian Boost that you link a bank account that you have automated bill payments with, and I hear it will help bump your credit exponentially rather quickly. I saw it on the news. It's not out yet, but if you go to experian.com/boost you can sign up and get an email notification as soon as the feature becomes available. I wish this was available back when I was focusing on credit building! Best luck and wishes. Its a rigorous uphill battle, but obsessive vigilance will suit you well in your endeavors! God speed.
Tip #1 by @KhongPhuTu is wildly incorrect. I am a banker by profession and I will tell you that carrying a balance between statements is ill-advised-- Don't pay interest! However, you should wait to pay the balance in full when the statement is cut. Statement balances are what is reported, not that you carry a balance between statements.
Raising your credit isn't just being responsible with one line of credit..although that is great to do. Opening lots of lines or at least 1 or 2 every few years and staying responsible with all of them really raises your score. Note: Don't open to many inquiries within a 2 year period, this may lower your score. Let some fall off before opening more. Let's say for credit cards..you get 5 or 6 over the course of many years and only spending at least one third or less of your limit and paying them off monthly on the due date. That's a good view and managing your money and you are establishing credit. You can also establish credit by taking out loans, such as a car lease or a mortgage. With these larger debts you want to be be sure you are on time or early with all your payments. Paying of any debt is also a great way to raise your score. Establishing credit takes time, hang in there!