Moderator's note:
Posts from new members seeking links to 'helpful' websites have a nasty habit of being followed by other posts from new members who just happen to know of such sites. (i.e. they're poorly disguised spam). If that happens here I'll simply remove the thread.
However, assuming this post to be genuine, I'll point out that Piggynose has already mentioned the most well-known site:
http://www.skyscanner.net . You need to be aware though that many airlines go out of their way to block third-party sites, such as Skyscanner, from accessing their fares. They'd much rather that customers booked directly through their own websites, so that they can be offered the 'extras' (such as insurance) which account for much of their profits.
So the best way to find cheap flights is to go to the website of your planned destination or arrival airport and find out which airlines operate on the route you're interested in. Then go directly to the websites of those airlines. (In practice the Wikipedia page for a particularly airport is often to quickest way to find out which carriers operate on which routes. The information can sometimes be hard to find on the websites of some airports).