.45 guy is right. No one knows for sure the formula the bureaus use to calculate your score, because they won't disclose it, but as far as I can discern, you should have at least three, but no more than six lines or revolving cred open, the longer the better. If they are maxed out, that hurts your score. If they have zero balances, that hurts your score. (You are being granted credit and not using it, 'abusing' lenders, and leaving open the possibility that you could go max them out tomorrow, and be in trouble.) It seems that if you have had at least a 33% balance on them, it doesn't hurt you. I just got the gift of a government travel card, which is paid off by the government when I use it, but goes on MY report, and I get to keep things like FF miles from it.
Quick version, installment credit looks better than revolving credit, but only if it's for certain durations and a minimum amount of time. If you go to a credit union, and secure a cash loan in the amount of at least $800 (meaning, GIVE THEM A DEPOSIT OF THE SAME AMOUNT TO SECURE THE LOAN), that you take at least 8 months to pay back, it will put a very positive mark on your credit. If your credit is messed up, there are services that offer to clean it up for you, for anything from a few hundred to a few thousand dollars, with varying degrees of effectiveness, BUT, it's nearly impossible for anyone to clean up any bad stuff in your credit report permanently if you still owe money on it. Of the things that ARE paid off, it's a lot easier to get it removed if it's at least two fiscal years old. NONE OF THIS IS GUARANTEED. Anyone who tells you it is is lying.
Credit is a mother-effer. Of course we should all be prudent and never go into debt for anything. But if you go your whole life without credit, (without having played the game,) and then try to get a mortgage, if you get accepted at all, you will get hammered. Now more than ever. You have to have it, use it, not OVER use it, and not listen to the lenders to know when you are doing something stupid, because it is not in their interests to help you.