Many online periodical indexes show you exactly how to cite the sources you find. Look for a link that says something like "citation tools" or "cite this article." Click on that link, and you will see the citation in various formats. Here is an example of a citation in MLA format created in Academic Search Complete.
Note: you are ultimately responsible for making sure your citations are correct. Make sure that your indentations and spacing are correct.
Medvigy, David, et al. “Simulated Changes in Northwest
U.S. Climate in Response to Amazon
Deforestation*.” Journal of Climate, vol. 26, no.
22, Nov. 2013, pp. 9115–9136. EBSCOhost,
The Purdue Online Writing Lab MLA Formatting and Style Guide.