At its most basic level, plagiarism is using someone else's work without giving them credit. Not cool.
The most common method of avoiding plagiarism is to make sure you correctly cite the resource whenever you summarize, paraphrase, or quote something you didn't write. The easiest part of this is citing quotes: If you've used quotation marks (" "), make sure you have a citation immediately after them (how you do that citation is determined by your style guide - read on!). Paraphrasing is when you take someone else's sentence and reword it in your own words with the same meaning; this is a useful tool, just make sure you cite that, too, since the idea is someone else's. Summarizing is a little harder because it can involve multiple sentences where you succinctly shorten someone else's idea; this still needs to be cited, even though you're using your words (it goes back to whether or not it's your idea).
When in doubt, cite it. If you're using someone else's words or ideas, give them credit. If you cannot say with absolute certainty that it is your original idea and words, then you should cite it to be safe.
Most databases (including SAGU Discovery) now have automatic citations available in any style you would like. When you find an article/eBook you would like to use, look around on the page for a citation link - you'll probably find one. Always make sure you verify the correctness of these citations - sometimes they are just a tad off. Having these citations to at least get you started is a great help though!
Reading the style manual for your discipline can help you be a better writer. Additionally, you'll want to verify with your style manual that the citations produced by citation makers or databases are actually correct; sometimes it's close but not quite right.