There's no such thing as a 'fixed cassette', you either need to use a new rear hub that's designed to take a fixed sprocket (that is, a screw-on sprocket with a left-handed-screw-on lockring), or you can use a product called a 'Surly Fixxer' which replaces the freehub on your rear wheel (and then install a fixed sprocket and lockring as normal). Unless you've got a really good quality rear wheel, it'd probably be cheaper to build up a new rear wheel around an appropriate hub.
You probably can't re-use your 9sp chain with a fixed gear drivetrain because fixed sprockets are almost always 1/8th width, which is wider than your chain - a 9sp chain is 3/32 which is too narrow. There are some 3/32 sprockets out there, though. You can use a 1/8 chain on 3/32 sprockets, mind, just not the other way around.
You could make it into a singlespeed pretty easily by removing your cassette and replacing the cogs with just a single cog, and filling the gaps with spacers. If you use a 3/32 cog, you can re-use your 9sp chain too (provided your front chainring is 3/32).
You shouldn't need a chain tension because you have horizontal dropouts. You can't use a chain tensioner with fixed-gear anyway (although you can with singlespeed).
This is a useful resource: