I am a fan of cheapflights.com and have picked up some great deals there. I also check out the budget airlines in the countries I am visiting like Westjet in Canada, Air Transaat between Canada and UK and Easyjet or Ryanair in UK and Europe. Good luck with cheap flights
I would look for the best airfare deals by google search the keywords "best airfare deals". The results will show the most ranked airfare sites and click on anyone of them. This is the best part to start.
I love kayak.com because it searches other search engines and links you to the sites with the best prices. You then book directly with the airline or site. It will also show you a 3 month range of differing prices to show the benefit of altering your itinerary.