The Impact Factor is a measure of the frequency with which the "average article" in a journal has been cited in a given period of time. It is one way scholars measure the relative importance of a journal within its field.
from University of Denver Libraries: http://libguides.du.edu/c.php?g=204307&p=1348066
1. Look for your topic in an appropriate database e.g. Web of Science.
2. Search for your topic e.g. sample Search: "clock proteins" and sleep (in Topic field)
3. Click Search.
4. Once on a results list page, click on arrow by Source Titles (in far left column).
5. You will see the journal titles listed which have articles on this topic. Do some journal names include much content in this subject area?? e.g. Chronobiology International, Journal of Biochemistry, Journal of Biological Rhythms etc.
6. You could take a few journal titles e.g. top 6 and then look up the Impact Factors for them to make your own rank order.
Find the journal's site online. Look for information about the journal. For instance, on the Cell site look for information For Authors > Information for Authors. This will take you to a paragraph about the Aims & Scope of the journal.