At an airport with an operating control tower (i.e., during operating hours), you can request FF from ground control before taxiing.
Otherwise, you'll have to wait until airborne to contact the nearest ATC facility (approach, departure, center, etc) on the appropriate sector frequency and request FF. You can find the appropriate sector freq any number of ways, including the A/FD, instrument approach plates for the airport from which you departed, on airnav.com, etc.
Some regions also put out ATC sector freq maps for the region. You could contact your local FSDO and ask if your region has such a map available.
Even if you pick the wrong sector frequency for the spot you are in when you initally call up ATC, they will typically tell you the correct sector freq to contact.
Such a scenario might be:
You: "Seattle Approach, Cessna 4999X off Bremerton runway 34, climbing 1,200 for 2,500 request flight following Redmond, Oregon via Portland"
Seattle Approach (wrong freq): "Cessna 4999X, contact Seattle Center for flight following on 128.5"
EDIT: 5 thumbs down? Tell me what I wrote above that is even remotely incorrect.