If you are a class/race who can forage, you right click one of the buttons on your abilities menu, and select the forage choice. Then you can just click that button to forage.
Druids and Rangers get forage as class abilities Iksar, and wood elves get forage as racial abilities (capped at 50, unless you also are a ranger or druid)
There may be others who get these abilities, but those are the ones I know for sure.
East Commonlands Everfrost Freeport East Freeport North Freeport West Nexus Qeynos Hills Qeynos South South Ro Warsliks Woods West Commonlands Western Plains of Karana