most people that convert to single speed use freewheel hubs (like you have) not cassette. you can remove your suntour freewheel with a special tool designed to do so, or have your LBS do it for you (it takes ~5 seconds).
you will need to buy a single speed freewheel, which can be had for $10-$20. the only problem this presents is spacing. if you are using the chainring on the inside of your cranks, you may be close enough. otherwise, you can respace/redish the rear wheel, or get a shorter bb assuming there is clearance.
i agree with the others who have said remove your derailleurs, shorten your chain, and run it as a ghetto single speed for a while.... that'll tell you if you like it enough to spend money on the conversion.