<p>I am working on a robotics project and need to print some gears. These will probably by under a LOT of pressure. Which material/filament should I choose so that the gears don't wear off easily?</p>

<p>PS: Newbie here...</p>

<p><strong>EDIT:</strong>
According to my instuctor, it has to be some sort of plastic (not metal).
It also has to be lightweight...</p>

So, as you say you want to materials for printing robotics parts. And as you have not given any budget constraint, I would give you a list of materials which would help you achieve the task, and you can choose amongst them accordingly.
Plastics: Basically used for building prototypes. Nylon Polyamide should be a choice for you.
Polyamide 3D printing is achieved through SLS 3D printing. It offers
  strong and flexible prints. The upside of this material is that the
  printing technology requires minimum preparation of the 3D file before
  printing. There is no need for support. And it also offers the
  possibility to create intricate shapes and moving part in just one go.
  After the print the polyamide can be polished and dyed.
Metals: Metals like Brass, Alumunium and Steel should be a good choice.
But, if I were to achieve your task, I would select carbon fiber. some details about it:
Carbon fiber consists of 90% carbon atoms, each fiber is 10 times
  thinner than a human hair. Carbon is especially prized for its lack of
  combustibility and infusability but also by its incredible strength
  (stronger than steel) and ability to create flexible structure, light
  weight and corrosion resistance. Its melting temperature is 1500, this
  heat there are only carbon.