Some amp have built in distortion. It is most often called "Gain". Your amp doesn't seem to have a gain control. That means your amp will produce a clean sound with very little distortion if not at all.
Like dvenetian, sometimes guitar player uses pedals to produce the distortion. If you watch one of my metal rhythm lesson, you will hear the distortion.
http://www.guitartricks.com/lesson.php?input=10014&s_id=231
I use a external effect to produce that sound. It's a Zoom G2.1U effect pad.
If you want to play metal (Like Black Sabbath's Iron Man) you'll need an amp that has Gain, or an external unit like a pedal (one effect) or an effect pad (multi-effect).