MLA? APA? Chicago? You're not sure? It doesn't matter at first, because they're all basically the same, no matter how different they seem. All citation formats include these elements of basic bibliographical description: AUTHOR, TITLE, SOURCE, DATE. Most also include electronic identification for items on databases or online.
SOURCE (publisher, journal info, "container," producer, website info, etc.)
DATE (location varies, but it's always required)
ELECTRONIC LOCATION (DOI, URL, name of database, etc.)
Immediately upon finding a possible source (article, book, webpage, or whatever), make a record of these four or five bibliographical elements, which are common to all styles and formats. Take notes, get a screenshot, check the book out, email the record to yourself, copy and paste the site URL, put it in a folder...if you can't describe the source, you may not ethically or honestly use it. Later, when you have time, format the information according to a standard. Below, the box "Style Manuals" lists handbooks for the styles most frequently assigned. If you get stuck, ask for help from a professor, a librarian, or a tutor.
Purdue University's Online Writing lab has been a nationally-recognized reference for conventions of formatting and citation, as well as many other issues in writing. However, it recently went into business with a for-profit company so is suggesting the use of sponsored monetized resources including a citation generator and advertising which may cost you money or trick your computer. Beware of these; avoid them.
Though its main content is still helpful, use this OWL's links with caution. Many databases also include a basic citation generator. Check the results, though; occasionally an error appears.
These books are the standards for style, format, and citation in their disciplines. Ask for them at the reference desk.
ACS (chemistry): QD8.5 .A25 2006 REF.
APA (social sciences): BF76.7 .P83 2013 REF.
Bluebook (law): KF245 .U55 2010 REF.
Chicago (humanities, history): Z253 .U69 2010 REF.
CSE (sciences): T11 .S386 2006 REF.
MLA (humanities): LB2369 .G53 2016 REF.
Turabian (Chicago for students): LB2369 .T8 2013 REF.