How should I go about researching my biological father's family line?

I'm 19, and not too long ago my Mother and my Father (divorced for 12 years) were suddenly sitting in my Mom's kitchen which is about 2,000 miles from my dad's house, so I knew something was odd from the beginning. They told me that my identicle twin brother and I were actually the biological father of another person. I can't say the same for my borther, but it really doesn't make a difference to me in any facet except my nationality and family tree. If I had had his last name, it would be Malone and I now know I'm half Irish 25% Scottish and 25% "English." Not sure if they meant mixed english nationalities or just English. Anyway, that's all I know at the moment, and I don't think I can just waltz into Sean's (Biological father's) house, whereever it is, and be all "HEY, WHAT'S UP?! So tell me about your family tree, now." Any suggestions on how to begin my search and where?