tools needed to put on a new rear cog
 

what do I need? I wanna switch my rear cog out. do I need a special tool to add/remove lockring?

You'll need something like this SPA_3 Spanner tool and then a chainwhip or vise(place the cog in the vice and then turn the whole wheel counterclockwise just like the bus driver) to remove the cog.

I really like the Hozan lockring tool I picked up - way more torgue and less slippage than the Park tools. No photo, but here it is for 19.95 at Harris :

http://harriscyclery.net/site/itemdetails.cfm?ID=530

Chainwhips are totally neccessary, but you might have to modify it. Most of the ones I've seen came with a 3/32 chain, which I had to change to 1/8 in order to really lock the cog down.