Song of Myself is not written in Old English, which refers to language before the Middle Ages (before 1400). Song of Myself was written in the 1800s. One way to try to understand it better, and just to learn to read poetry better, is to keep a dictionary handy, and look up the words as you go along. After looking up the words, try paraphrasing the line in your notebook, and you will be pleased with how well you can begin understanding the poem. This, together with the information enotes provides, will give you the skills to understand poetry more easily in the future. Some of Whitman's allusions are difficult, however, and at these moments enotes might be more essential to understanding the poem.
Enotes has a complete summary and analysis of the poem. Just follow the link below, and you should be able to understand the poem better in order to write it in prose.