Academic sources
Information literacy
If you regularly cite random webpages you find via Google, then you need information literacy. Howard Rheingold aptly calls this 'crap detection'. Don't cite crap! Use the CRAAP test (originally by Sarah Blakeslee, checklist by RDC library) to ensure that you are citing only the good stuff (PDF version).
Wikipedia
While you will want to avoid citing Wikipedia entries, they are often a great way to orientate yourself to a topic you aren't familiar with. In addition, the references cited in a Wikipedia entry are often reliable sources that are worth checking out (though the onus of evaluating them is still on you). Wikipedia itself has some advice for academic users.
Academic journals
Journal articles are often more up-to-date than books. In many cases, important studies or issues are first published as journal articles. Google Scholar searches through major journal databases as well as Google Books (see below). Some articles are open source and therefore free, but unfortunately most are still behind paywalls (i.e. you or your library need to pay for access). Google Scholar will tell you if a free alternative exists on the web. If you add your university library (Settings > Library links), it will also give you the direct link to the article through your library portal, so that you can access it once you log in. (You may have to repeat this process from time to time.)
Here's a short tutorial from a university.
The easiest way to use Google Scholar is to install the Google Scholar Button on your Chrome.
Another useful feature is the cite function, which formats the reference for you (but beware of mistakes -- always double-check).
Books
Many books can be previewed on Google Books. Normally only a limited number of pages can be viewed; make good use of these by looking at the contents page and using the search function to search inside the book. (If you run out of pages you can view, you might in some cases be able to 'reset' this by signing out, using an incognito window or using a different Google account.) Depending on your needs, you may find this enough, or decide that you need to retrieve the book from your university library (if available). Your library might even have the book as an ebook. Google Books links each book to its WorldCat page, where you can search local libraries for the book. Click on Find in a library to go to this page (you may need to click Get this book in print to see this). If this link is not present, search WorldCat directly.
Here's a tutorial.
I can't find what I want!
No access to a university library, or can't find what you want on yours? Try NLB, which has some scholarly eResources for its members (open to Singapore residents only).
If you still can't find the article you need, you can use the Open Access Button or unpaywall to help you search for a free copy, or rely on the kindness of strangers by requesting for the article on Twitter, using the hashtag #icanhazpdf (remember to protect your email address, delete your tweet after you've got your pdf, and never thank the sender in public). The latter solution does run the risk of infringing copyright -- you've been warned.