HereForTheBeer: For online, credit card. In-person, either.
Sounds like you're just starting out, and the temptation is to go nuts with a credit card. A good option is to get a starter card with a small limit; $500 is typical for a starter card (sometimes called student cards).
For the credit rating your payment history is king. The bureaus don't care much if you charge $30 or $3,000 (some exceptions, ignore them for now) or how often each month you use the cards, so long as you make your payments. Only charge a little bit each month and pay it in-full every time, on time. Do that and you'll come out just fine, and will also start off with the good habit of not over-extending yourself beyond your ability to pay-in-full each month.
And really, don't put too much stock in your credit score. It's going to grow by itself if you keep your nose clean. Hell, you don't even need to use credit to have your score increase: lots of other places report on your payment history including some utilities and even some landlords (mostly if you're renting from a large property owner). Again, payment history is the big deal.
For now, just learn to be careful using these things - the ease of the swipe also makes it easy to get into trouble. But at least you're asking, which is more effort than a lot of people bother to put into it.
hedwards: That's mostly correct, but don't forget about credit utilization, banks want to know that other banks have trusted you with the ability to borrow money and that you've restrained yourself far below what they were willing to give you.
Which to an extent seems kind of silly to me, why extend so much credit that making use of it is a sign of a problem? Granted for an auto loan or mortgage that's somewhat understandable, but for loans that are purely discretionary?
That's what I meant by the "(ignore that for now)" bit. Utilization is so friggin' goofy: for a higher score we want you to have this giant credit line... and then use very little of it. Which is a bad idea because that's exactly the sort of crap that leads to, say, housing bubbles and bursts, as people see how much credit they've been extended and then do something completely unaffordable with it.
You're right, though. I just didn't want to confuse the issue for the OP, and then get people thinking that having a high credit line is a good idea.
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So here's a really stripped-down explanation of what hedwards and I are talking about. The credit bureaus "reward" your score also based on utilization of your existing credit. Keep in mind that they don't know how much you make - the basics of what they know are: how much you owe on all credit accounts (they actually have access to monthly balances if your creditors report them), your credit limits (in the case of credit cards and equity loans) and whether or not you pay your stuff on time (payment history). Again, I may have left out some details but that covers it for this discussion.
So utilization goes like this: you have three credit cards out there, and maybe a home equity line-of-credit (HELOC). All told, maybe they total $29,000 worth of credit available to you to blow on stupid shit like a Hello Kitty shopping spree. Whatever. But you've been smart, see, and from one month to the next you're only using about $1,400 of that, and much of that is because you put your cell phone and electricity bills on the card, and also use it for gas and food and stuff for monthly college living. You know, the crap you gotta pay every month anyway. In other words, you're not stacking more and more debt on top, up to the limit of the credit lines. So the utilization part is basically a ratio of credit used versus credit available, and at $1,400 spent of $29,000 available to you, your utilization is low. The credit score rewards this because it shows restraint: you COULD go on that shopping spree and carry a big balance but you haven't.
Now the shopping spree in and of itself isn't a huge deal. It's when you can't pay off that shopping spree that it starts to cost you (interest charges). And then when you do that over and over again so your carried balance (your credit card debt) keeps going higher and higher, that's when it gets worse. For one, you're paying more and more interest. So let's say you've charged up stuff over the last year and now your total balances are sitting at $16,000 out of that $29,000 available. That shows that you're using a whole bunch of that credit available to you and it affects that utilization part of your score negatively. Lower utilization (lower carried balance) means better utilization, and that bumps your score a little quicker.
That does NOT mean that you should just get a bunch of cards and sit on them, just to help your utilization with a higher total available credit. The problem of that, as alluded to earlier, is when you get to thinking, "Ahh, I've been doing alright so far so it won't be a big deal if I carry a balance for a while." And maybe that works okay with a small balance so you figure it's alright to add more to it. And add more to it. And then you realize, "Shit, I'm spending $86 a month just on interest alone!" And if you're late on a payment a few times - or miss some payments, or can't make the minimum payment - then your score takes a hit AND you're probably going to be socked with a higher interest rate, so now that $86 a month in interest becomes $99 on the same balance. Makes it worse, so don't do that.
Anyway, don't fixate on the score. It'll mostly take care of itself if you don't do anything stupid with your credit and bills.
There's some decent explanations of this stuff here:
http://www.myfico.com/CreditEducation/CreditScores.aspx Note that this is a site where you can buy your report and score, but just use it to learn what this crap is all about. You could also learn at Suze Orman's site, clarkhoward.com, and Dave Ramsey's site. A bajillion places, really, but I trust those four.