There are several different ways to track flights online. You can do this through a flight tracking website, through the airport website, or through airline websites. This article will show you quickly how to do that along with a few examples.
Gather the information of the flight that you are tracking. What airline is it? What's the flight number? What airport is it taking off of and landing at?
Track using a flight tracking website. One example of this is http://flightaware.com/
In the middle of the left hand side of the page - you can track both airline flights and commercial flights by putting the information into the panel and selecting search. This website is great because it shows you an on screen display - and how far off the plane is from their approximated travel time.
Use the Airport to track flights and their status.
For example, Boston's Logan Airport has a page directly related to tracking flights (both incoming and outgoing).
http://www.massport.com/logan/tracker_screen.aspx#
Try the airline directly.
Some airlines (like Jetblue below) have a tracking service online you can follow.
http://www.jetblue.com/flightstatus/
Google the call sign of the flight - and most likely you'll be provided with a few places to track the flight.