What is flight following?
While VFR pilots are always responsible for seeing and avoiding other aircraft, ATC can provide a helping hand through the use of radar advisories, also known as flight following. On a workload permitting basis, ATC will call out the position of nearby traffic, aiding the pilot with his/her job of not crashing into anyone. The controller will also assist you in locating your airport, if need be.
How/when do I get flight following?
Aircraft departing, arriving or transitioning Class B fields (LAX, LAS, SAN) and Class C fields (ONT, BUR, SNA, SBA) are provided with radar advisories while they remain inside that airspace. Leaving the airspace, however, ATC will ordinarily terminate the service unless the pilot indicates he/she would like to continue such service in advance.
Aircraft outside of Class B/C airspace should contact the appropriate approach/center controller, stating their aicraft type, location, and request (who you are, where you are, what you want.) For example, "Socal approach, Cessna 132KT, 20 southwest Long Beach, request flight following to John Wayne."
The controller will issue a discrete beacon code so he can locate you on radar. "Cessna 2KT, Socal, reset transponder, squawk 0432"
Once you enter the code, and once the controller has time to find you (remember, this service is provided on a workload permitting basis, and ATC's primary responsiblity is to keep IFR aircraft from banging into each other), you'll hear "Cessna 2KT, radar contact 19 southwest Long Beach, John Wayne altimeter 29.92"
You will now receive basic radar service from the controller until you explicitly cancel the service, or the controller notifies you that radar services are terminated. The latter can happen you leave the controller's airspace and the next controller isn't available, or is unwilling to provide the service. It can also happen in the real world if you enter an area with poor radar coverage, although that is not an issue on most online networks.