How do you take off cogs?

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So my dad found a Mongoose Switchback in a lake, and I want to convert it to a singlespeed. I took of the rear wheel and attempted to take off the cogs, only to discover that I don't know how to do it. I thought it would be easy, but I tried everything and I can't get a single one off. What am I missing here? I saw pictures online and it looked like they just slide right off. I'm confused.

In a lake? Perhaps it was being used as a boat anchor. Better throw it back.

So my dad found a Mongoose Switchback in a lake, and I want to convert it to a singlespeed. I took of the rear wheel and attempted to take off the cogs, only to discover that I don't know how to do it. I thought it would be easy, but I tried everything and I can't get a single one off. What am I missing here? I saw pictures online and it looked like they just slide right off. I'm confused.

they are held on by a lockring which you need a chainwhip and lockring nut to take it off with. Have a look at the end of the cassette, should be a small ring with lots of little teeth on the inside, that's what you need to get off.

first you need a lock ring tool
http://www.parktool.com/images/products/productimages/spr_HCW-5_2005630_79816.jpg

and then a chain whip

http://images.amazon.com/images/P/B000BB8FW8.01-A369ZTW9RUY8F0._SCLZZZZZZZ_.jpg

(http://www.stlbikeworks.com/store/shop.php?c=1&n=3404731&i=B000BB8FW8&x=Park_Tool_SR_1_Sprocket_RemoverChain_Whip)
Should be pretty obvious from there... remberber the lock ring is threaded backwards...hopefully the person who put it on before used some greese...