FreeCreditReport.com does a great job of pointing out issues and common ways to fix them. They also explain how things impact your score and ways to use them to your best advantage. Like credit cards. They're good to have, bad to max out.
All credit cards will help if used responsibly. I started with Gas cards. They're easier to get into since they don't have huge limits, and you can't go wild with them since they're only good at gas stations. Use them then pay them off each month.
Store credit cards work too. I have a Best Buy card and carry a balance that I pay down making at least double the minimum payment each month. And Best Buy usually has "Same as Cash" deals with their card where if you pay off before a certain date it's 0% interest. Just be SURE to pay off by that date, or they'll hit you with ALL the interest you skipped.
If you think you can be faithful and not fall behind, get a car loan. Even if your credit is bad, most dealerships will be able to find SOMEONE to give you a loan. It'll be at a really poor rate, and you'll have trouble sitting down for a while, but next to a house loan it's one of the best things for your credit.
My score is getting up there, but my report says I'm being hampered by my lack of big credit cards. So I picked up a card from my bank that I'm going to use like my Best Buy card. Once I start using that, I hope to get close to or crack 800.
All of this advice is worthless if you don't keep on top of your payments, however. Good bill paying history is essential.