start with facebook ... if you know dads first and last name you can often find many relatives there
I think walkover means Heritage.com
I'd start with court records: birth certificate will have mother and father's names, or marriage certificate (if your mother was married to him). Maybe, run a background check on your dad and it should include older addresses and such. Right now, there are a couple of TV shows airing in the US about people searching for their family ties. On Friday night - NBC: Who Do You Think You Are? and on PBS, there's a similar type show, but I don't remember the name.
If you know the city or town where he was born, you can use that as a starting point. Good Luck!
GO TO MY HEARATAGE .COM sorry bad spelling but you get the jest,,