That depends on your personal credit, honestly. The first thing you should do is pull a credit report from AnnualCreditReport.com - they will give you all three credit bureau reports for free. You will want to carefully analyze this for accuracy. If you see something on there that shouldn't be, dispute it. If you know you've paid something, dispute it. I raised my own score almost 100 points by taking this action. You can also have accounts removed if they are older than 7 years - great idea if you have had problems in the past. This step alone is the fastest thing you can do to improve your score.
While going through this process, do NOT apply for other credit. While multiple inquires will only ding a few points, if you are buying a house or trying to secure financing sometime soon it can cause problems.
Make your payments on time! This is quite literally the biggest factor in your credit score. If you don't make those payments on time (no more than 30 days late), you will always have bad credit. Set up reminders on your computer, your phone, wherever is most convenient for you.
Diversify your credit with a mix of revolving and installment accounts. Some sources recommend 3 open trade lines. Truthfully, it really depends on what you are trying to achieve by improving your credit.
The first step is pulling that credit report. I highly recommend starting with that from personal experience.