Great question! Also let me commend you for being in good condition, financially, at just 20 years old. 2 years ago you would have had a lot easier time than today, but most lenders are going to require 3 active credit lines in good standing.
That said, what is a credit line? Of course credit cards, auto loans, personal credit etc, which you said you do not have.....but what about: Have you paid rent to a landlord or servicing company? Do you have a cell phone in your name that is on a contract-not prepaid or MtM? Do you have any bills in your name-BGE, Phone, Car insurance? Some lenders will use these as trade lines if you have proof-or a statement-from them saying you have paid on time.
If you have none of these, then you will have to establish some trade lines and pay on them for 60-90 days as Dan said. Your best bet is to talk with a good lender in the area and they will help you to establish what you need. If you need some great suggestions for lenders, contact me and I will pass along some recommendations.
As a side note, if any of the above options are possible with trade lines and you can find a home, you could still qualify for the $8000 first time buyer tax credit!
Don't hesitate to contact me if I can be of any firstname.lastname@example.org or 443-912-4866