Beowulf has unusual MLA citation for a few reasons.
1) It's anonymous.
2) It's published as a book on its own when it isn't in an anthology, so it gets italics, not quotations around it like a smaller poem would.
3) It's a poem.
4) It's (of course) a work from an anthology.
1) Since there is no author listed, the reference for Beowulf goes on your Works Cited page listed under B. A parenthetical reference after a quote would use the title and line number like this:
Beowulf the brave warrior has "the strength of thirty men" in each of his hands (Beowulf 535).
2) Since it would usually be published on its own (as a full-length story deserving its own book), it goes in italics when you are talking about the book's title (like the parenthetical note above). Of course, if you are only mentioning Beowulf the CHARACTER in your sentence, that would not get italics (again, this is illustrated in the sentence above).
3) In the citation of that quote above, 535 is the LINE number. Since Beowulf is technically a poem, you cite it by line numbers. If you want to know more about the rules for "citing poetry in MLA format," there is a link on the main course webpage to help you out with it.
4) On your Works Cited page, it would be listed alphabetically under B for Beowulf, but the publication information, etc would all be for the anthology. There's an example of a work from an anthology on the sample MLA paper on the filing cabinet link of the main course website if you need more help with that.
Hope this helps.