Check if it's freewheel or freehub.
Cassettes you use two chainwhips.
6/7/8 speed all the same*
One to hold the large rings pushing 'forward' aka clockwise
One to I unscrew the smallest ring 'backwards' aka anti-clockwise.
It may not undo
Freeweheels you get the right tool, thought two holes is different to the ones I've seen. But some may have used them.http://www.yellowjersey.org/stfw.html
*7/8 speed cassettes with an 11tooth aka MicroDrive cog are a bit different, you unscrew from the 2nd to last cog, not the smallest. But you'll not have that.
Edit to add.http://www.yellowjersey.org/ST_CASS.JPG
I'll scan in a better copy. It's sat next to my scanner with other stuff but I just need to do it sometime.