Historicism is a school of literary criticism, not a type of poetry or characteristic of poems. Thus what your instructor is probably asking you to do is to investigate how one would go about applying historicist criticism to "The Love Song of J. Alfred Prufrock" by T. S. Eliot.
The starting point of such a critical approach is to look at the nature of the poem's historical context. Eliot was an American poet living in England. He published the poem in 1915, in the midst of the horrors of World War I; although the war was not being fought on English soil, many English soldiers were experiencing the horrors of trench warfare and the news was filled with stories of the war. The character Prufrock is middle-aged, too old to be a soldier and thus removed from the traditional masculine wartime role, and surrounded by the women of London society.
Next, Eliot was concerned about the way that modernism was changing the world as industry was transforming the landscape (the great fogs of London) and science was alienating people from earlier modes of thought. Thus Prufrock himself is both part of an older society, formal, precisely dressed, and nostalgic for mermaids, but living in a modern world and can neither go back to an earlier time (he is not Prince Hamlet and the mermaids will not sing to him) nor fit successfully in the new modern world.