Depends on if you're a loader or an unloader. Unloading is easy, just grab a box and chuck it down the conveyor belt. Loading is another story. You have to memorize several zipcodes assigned to whatever truck you're on, make sure each package is scanned correctly (make a mistake and someone loses a package for a week), the STACK them as fast as possible before the conveyor belt backs up - wich it always does - like a non-stop, intense game of tetris.
Both of these jobs involve INTENSE physical activity, so be prepared to sweat bullets in places you didn't know could sweat.
Then there's the Smalls. These are packages that are too small to be stacked individually, and need to be placed in bags. This is a less stressful and physically demanding, since you're pretty stationary and the job ends as fast as you can get everything done, so you can essentially work at your own pace (sort of). The only downside to this is that there's still the matter of accuracy, a line of packages that never ends, and possible boredom. Also, this position is a little harder to get for a beginner.
Hope that helps. I myself couldn't handle the intensity of the job, but that's just me. You might fair better. In any case, if you decide to take the job, take full advantage of the benefits, wich is pretty much the only reason most people take the job.
Good luck, and don't fuck up your fellow anon's mail orders.