Discuss Blake's use of symbols in Songs of Innocence and of Experience.
This is a pretty big topic, and the first thing I would suggest is to narrow down your topic to a smaller thesis, as Blake uses several symbols and Songs of Innocence and Experience covers a lot of territory.
One recurring symbol that Blake uses is that of the Lamb, not only in the poem of that title, but in several of his other poems from that series, such as the Chimney Sweep. Blake's lamb is a multi-layered symbol in that it represents innocence, children and Christ all at the same time. You might want to frame your paper around that, using the poem "The Lamb" as your starting point. Then, you can look at how he uses the same symbol in other poems in the series.
You don't indicate if your essay is a research paper or just a response paper, but you can create the basic structure of a response paper by using the three intepretations of the Lamb and elaborating on each one as it occurs in several of the poems. Then, if you need to add research, you can probably get what you need from the databases at your particular college and hang your supporting research on the framework you've already created in your response.