This is a very interesting question. The short answer is that it depends on what country you are writing in. In British and Australian English, one typically uses single quotation marks. If writing in North America, double quotation marks are typically used.
I’ve noticed some students adopt another rule: double quotes for long quotations, single quotes for single words or short phrases.
Double quotes are used to mark speech, for titles of short works like TV shows and articles, as scare quotes to indicate irony or an author’s disagreement with a premise. In America, Canada, Australia and New Zealand, the general rule is that double quotes are used to denote direct speech.
Single quotes are used to enclose a quote within a quote, a quote within a headline, or a title within a quote.
In the UK, whether to use single or double quotes to mark direct speech depends on editorial style, but both are acceptable.