Despite being made to feel very unwelcome by some I shall try and give you some advice mohammad034.

The easiest way to find the cheapest fares on a route is to use the price comparison sites like Kayak.com. They will direct you to whichever agent or airline has the best price.
There are other online tools that you can use like ITA but in my experience they usually come up with the same results. To be honest there is no magic formula - if the cheap fares aren't there in the first place you won't get them no matter what you do.