Knock on the door or call the number on the sign. Ask how much and ask if they'll work with a real estate agent. If they will pay a realtor commission - find a local realtor, ask them to assist you. you don't need a real estate agent, but they help to help you establish market value - so you don't overpay for the property. And actually a professional appraisal will help you determine that - but you also need to know the market values to make a good offer. FSBO's generally overprice their homes - dramatically in some cases.
Also - if you're not preapproved for a mortgage - do this FIRST (assuming you'll need a mortgage). You can also find out what kind of mortgage the owner has and possibly assume their current mortgage, especially if they have a good rate. Most mortgages do not allow the loan to be assumed, but there's ways around that and it can be negotiated with the lender. if they have no note on the property, then you might be able to have the owner hold the mortgage - check with a real estate attorney (note a REAL ESTATE attorney) to find out details about the law in your state.