How do I know whether my bike can use disk brakes? I have a Gary Fisher bike (wahoo I believe). I think it's considered the entry level Fisher mountain bike. 
Could I put on disc brakes as opposed to using the standard  brakes on this bike?

Standard brakes are the rim brakes. 

Thanks
Carlos
You have to check if the frame and fork support this. That is, are there a couple of holes where the disc brakes are supposed to be, like the ones in this picture: &nbsp;


  


You will need to change the front and rear wheel, as your current ones probably do not have the six mounting holes for discs.
You may consider exchanging only the front brake, as it is generally more often used. Leaving the rear brake a V-brake will help with not adding so much weight to the bike (and expense).