What makes one source more credible than another?
If you're referring to articles and other things you find on the Internet, here's a good site to help you learn to tell the difference:
The Good, the Bad, and the Ugly
http://lib.nmsu.edu/instruction/eval.html
Click on Criteria and read it through; then try your hand at determining which is best, which is worst, and which is "iffy" in the Examples section.
And here's another:
http://www.sou.edu/library/searchtools/evaluate.html
If you're referring to books and other sources in print in your library, be sure to consult with the reference librarian. The reason we employ librarians in all libraries is to weed out the awful from the good.
The author needs to be credible -- that is, he/she is not just some joe-schmoe from wherever!
The publisher/sponsor needs to be authoritative, too -- a university or a government or some non-profit sites are usually far preferable to any .com websites.
There should be some indication of when the site was last updated -- and it should be relatively recent.
The topic and the stance should not be "off the wall" -- remember that just about anyone can publish just about anything on the Internet. You have to be careful.
=)