I just wrote a hub on this as it's a bit complicated to answer in a small area such as this. My approach was one of repairing credit, but the same methods work for building it.
The one thing to remember is that it takes time to rebuild your credit just as it does to build it - and that the quickest way to build credit is via a legitimate secured credit card (issues by a bank - not a merchandiser). These cards are easy to get (as they are secured against the money you pay for them) and if used wisely (for small monthly purchases totaling less than 25% of the card's value) and always paid on time (prior to the billing date), they ca raise your credit score quite a bit in just a year's time (in my case, enough to purchase a house).
For those wanting to know more about how your credit is scored and how to repair it, I wrote up this hub - enjoy! :)
And to dispel a myth ... credit companies are not interested in how much you make, so much as they are concerned with how well you repay your debts. ;)