I expect it's a freewheel not a cassette. Can you see any writing on the saying it's brand. You will need a freewheel remover. http://www.parktool.com/products/category.asp?cat=4 shows a selection of tools for different freewheels.
Once off you can screw on a single speed freewheel and redish the wheel. What your chaining will be like I don't know. You should be able to get a nice chainline with the dishing.
I expect it's a freewheel not a cassette. Can you see any writing on the saying it's brand. You will need a freewheel remover. http://www.parktool.com/products/category.asp?cat=4 shows a selection of tools for different freewheels.
Once off you can screw on a single speed freewheel and redish the wheel. What your chaining will be like I don't know. You should be able to get a nice chainline with the dishing.