As a general matter, your initial regular commute or trip to work is unpaid; however, once you get to work, anything you do for your job (i.e. not for yourself, on a lunch or other break) would be work time and should be paid--and that includes driving to a job site. Had your employer had you, for example, take home supplies or equipment the day before, then drive straight to the job site from your home in the morning, in most situations that would be unpaid time (so long as the trip was not significantly longer than heading in to the office); but by having you come first to work, the employer would seem to make the subsequent trip to the job site (and loading up the truck) into work time, for which an hourly employee should be paid.