As @Anthony Beckemeyer said, you should pull comps. If this is the first time you have done it, it may take a while to sift through all the information. If you're unsure of the value, ask a real estate agent or another investor to verify your numbers. You'll need access to the MLS to get the most accurate comps... don't rely on the tax appraisal values or Zillow, etc.
I would also drive by the property if it isn't too far out of the way. You'll get an idea of the condition of the house.... chances are, if it looks rundown from the outside, the interior is just as bad (though you won't know for sure until you see the interior). You'll also want to get a feel for the neighborhood and the area.
Also, don't forget to follow up! I know it sounds simple, but it is crazy how many people put out ads and never answer the phone or follow up when you call.
Good luck and keep us updated!
