Pizza in Oven : Bottom/Mid/Top
Ideally, of course, you'd be baking your pizza on a baking stone, which you would heat to 500F for 1/2 hour before putting the pizza in the oven.  However, you asked about baking a pizza in a metal pan.

In general, you want to get as much radiant heat into the bottom crust as possible in order to make sure the crust is fully cooked and not soggy.  This means that in an oven with a bottom baking element (whether gas or electric) you want your oven rack in the bottom position.  Again, make sure the oven is thouroughly heated to 500F (250C) (or more, if it'll go higher) before you put the pizza in.

If you have a top-heating oven, things get a bit more complicated.  You'll need to somehow ensure that the bottom of the crust gets cooked, which won't happen if you just put the pizza in the oven, regardless of position.  There's a couple of different ways to take care of this:


Blind-bake the crust until halfway done, flip it, put the toppings on top and finish it, 
Do the pizza in a cast-iron pan first on the stove top, finishing in the oven.


In either case, with a top-heating oven, you want to put the rack close to the element ... in the middle or top position, so that the pizza is only about 2-3 inches (5-8cm) away from the element.