If you're taking ideas from other people (which you should be - the assumption is that you won't say anything original in your first year at least), then you reference them if you're paraphrasing. If you're copy/pasting, the text should be in quotation marks as well as being referenced. Although if you do a lot of direct quoting, you may fail for having too few of your own words in the essay. You shouldn't just write an essay out of your own head, then go looking for references afterwards.
If you don't understand referencing, ask your course leader. It's usually one of the first academic writing lectures in any undergrad first year, so if you missed that, you'll be completely adrift.