Since your essay/paper will probably involve narraation, you may wish to recreate with as much vivid detail as you can how your ancestors entered the United States. For instance, if your ancestors came to America via Ellis Island, then you could, perhaps, recreate what the experience was like. Doing some research on this port of entry for an immigrant would be helpful in creating vivid descriptions.
Or, you could begin in one of the ways suggested by the link below which gives instruction on how to write an interesting introduction. Still, if you will make the first of your family come alive on the page, then yours should be an interesting paper to read.
First of all you create an atmosphere of Magic Realism by starting with sentences like: Once upon a time orLong long ago there lived oneMr.X in the village /town****.This will create interest on the reader.Then you may use flashback as well as flash forward technique to give the story mobility.