Start with www.google.com/flights. You can change departing or arriving airports without changing screens. There is a “dates” tab and a “chart” tab to compare. Most flights have different flight numbers for each partner airline. Sometimes the prices vary by how the flight is coded. Sometimes this variance is substantial. Another great thing about this site is you can add a broad destination. Say you want to go to Paris, simply enter “Europe” instead of Paris. You may find a flight to Brussels, Belgium for $500 less! It is a short 2-hour train ride from Brussels to Paris. Trains are relatively inexpensive in Europe. If you book a few days out the ride would probably be less than $50. So, now you have saved $450 AND have the opportunity to see another city. It is a win, win!