The first question is, do you really need all of the quotation? It's very easy to let sources take over your writing, when what's most important is your own thinking. It's best to quote smaller pieces--introducing them with a signal phrase (such as "So and so writes" or "Author X states"), then following with the citation and, most important, your explanation of how the quotation helps develop your argument.
However, if you do need a long quotation, the format depends on the style you are using--but if you are writing for a class, if the essay is double spaced, the quotation should be double spaced as well. MLA and APA have different rules for how long a quotation needs to be before it is set in "block format": For MLA, the issue is how many lines of your document the quotation takes up. If it's more than four lines, use block format. For APA, the issue is word count: if the quotation is more than 40 words long, it takes block format.
To "block format" a quotation, begin the quotation on a new line, indent the entire quotation 1/2 inch from the left margin only, do not use quotation marks unless they're in the original, and put the citation after the end punctuation.
If you do use block format, remember that after the quotation, you still need to provide your explanation for how the quotation helps your argument: your explanation is a continuation of the same paragraph, so start on a new line after the quotation but flush left, not indented.
A rule of thumb is that your explanation of the value of the quotation should be longer than the quotation itself: that's a check to help make sure the quotations aren't speaking for you but that your voice still dominates your essay.