What are the important elements of the source? -- author, title (journal and article if needed), publication date, publication info, electronic retrieval info (such as DOI or homepage of the journal or website, if applicable)
What format of source is it? -- electronic vs. print. (Did you get it from a computer originally? Even if you printed it off, it's still electronic!)
What type of source is it? -- magazine, journal, newspaper, regular book, reference book, video, audio, picture...
Using #2 and #3, find the closest-matching source citation example that you know is correct. Use your elements from #1, and make your citation match the example. Use examples from here: