Do I need to buy Fallout 3: game of the year to play broken steel?

No, you don't need to buy any special edition or anything. You could just go out and get 1600 microsoft points (sold at local game retailers if you didn't know) which costs 20 dollars. This'll be enough for you to get both of them, if i may say though i wasn't a huge fan of Point Lookout; Broken steel wasn't that bad.