I've been using CreditKarma for a few years, and it seems to have a pretty good algorithm.
Not only does it have a decent algorithm (my actual score was within 5 points of CK's estimation), it also has a good section that explains the different factors, how important they are, and where you stand. So at a quick look at my page it explains...
- the three highest factors that impact your credit score are: Credit Card Utilization (total debt* over your total limit), Payment History (whether you've ever made any late payments), and Derogatory Marks (for example, something sent to collections).
- Age of Credit History has a medium impact, and will likely be the highest negative contributor to you. My score is "Poor" simply because 1) I only opened my first credit card five years ago, and 2) every time you open a new card, it lowers your average age. But it's only a medium impact, and there's honestly nothing you can do except wait for time to go by, so no use worrying about it!
- Total Accounts and Credit Inquiries are of low impact. Don't worry about Total Accounts - I have 10 and that's deemed "Poor." They recommend 21+. But don't go opening cards and accounts for the sake of it. This does not have a big part in your score. Likewise, inquiries are also not a big deal, but I'd avoid opening any new accounts for about a year before you make any HUGE purchase (like a house).
As a mustachian, the first three factors that have the highest impact are pretty easy. I'd open one or two cards now and just be sure to pay them off every month!
* It's important to note what kind of 'debt' I mean. You should and will pay off your card in full every month, but each credit card company reports your debt at different times. So for example, I pay off my AMEX card in full on the first of every month. My statement closes on the 27th. AMEX might report what I owe on the 15th. Or the 26th. Or the 3rd. I don't really know. Unless you pay off your CC every day, don't worry about this. "Excellent" is 0%-9% utilization. The only reason I'd be worried about this is if you get a student card with a relatively low limit and you have expenses that end up higher than 10%. Considering my first student card had a limit of $900, it is possible.