One of the challenges of citing sources in your research is that new types of sources emerge faster than rules for citing them properly are formulated. We have a research guide, How to Cite Print & Electronic Sources, with examples of how to cite most common sources. In our reference collection, we have style guides such as MLA Handbook for Writers of Research Papers (LB2369 .G53 2009).
But what about citing tweets? Twitter wasn't much of a consideration when MLA was last updated in print, but the Modern Language Association answers the question of how to cite tweets on their website.