essentially, the most important aspect (mechanically) of a singlespeed drivetrain is is chain tension. the chain must be tensioned properly or it will derail, or fall off, the gears. if it's too tight, it will bind and cause excess friction and wear and the moving parts of the bike.

the problem with making this conversion is that most geared bikes now come with what are called 'vertical dropouts' which place the rear axle in a very defined and specific coordinate in the frame....

good for brake pad alignment and service... bad for tensioning a singlespeed drivetrain.

you could get a new rear wheel or relace the rear rim to a singlespeed hub, or you could take the gear cassette off and put a singlespeed freehub cog with spacers on, but either way you will be stuck with a cog that cannot be moved in relation to the chainring, cranks, and pedals.

this means you'll need either a specific gear ratio that will likely not be the ratio you want, or you'll need some kind of tensioning mechanism like a ss chain tensioner(either cheap and worthless or expensive and good but still added complexity to a system that is meant to be simple) or an eccentric rear hub(ex-****ing-xpensive!!!) and you'll basically have a bike that wants to have gears and doesn't.

the least you can spend on this would be about 35$ if you buy new parts, and that will allow you to buy a 30 dollar chain tensioner and cog kit that slides onto your freehub splines, plus a set of cheap single chainring bolts that will allow you to run one gear in the front instead of your three.

basically, you will be cutting the range and adaptability of your bike and effectively neutering it.

if you want a singlespeed i would test ride a fixed gear like a specialized langster steel, a globe roll, a trek district, giant bowery, or a raleigh rush hour. see if you like a more agressive bike with drop bars or a more aggressive road bike geometry.

if you do like the lighter, more rocket-ship feel of a long, stiff road bike that lays you out over it, you'll know that you would be better off selling your rapid and using that money to fund a new fixed gear singlespeed bike like the abovementioned.

if you're set on that bike with it's fun design and quick, light(ish) aluminum frame but really want a singlespeed... you can do it, but i'd hold on to ALL of the parts you take off of it, because no one in their right mind would want that bike as a singlespeed. you might as well have a singlespeed townie or beach cruiser iwth that bike's geometry and flat bars and tire clearance. it's a commuter, and it needs gears for the days you decide to take your briefcase or start loading up panniers.