Fixed gear

it's fairly easy, it depends on how into it you wanna go, you can PM me, or IM me on aol at empiresashes if you've got specific questions, but the general steps.....

easier way.....

take off all your derailers, remove/find a shop to remove your freewheel, put a fixed sprocket on, probably with some loctite or jbweld to hold it tight, I've heard bottom bracket lockrings are the same size as freewheels, so it'd be good to get one of those, but I can't say for sure....then you'll need to adjust the spacing on the axle, there should be a big spacer on the freewheel side, if you take that and put it on the other side, re-dish your wheel so the rim is still centered after you move your hub to the right(make sure you take the tire/tube/strip off the rim if you're gonna adjust the spokes that much, or it'll just pop your tire), and then just take the extra links out of your chain,
$15-freewheel tool
$15 chain tool
$15 sprocket
$5 lockring/jbweld, whichever you can use
$5 spoke wrench
close to $50, less if you find a shop that'll remove your freewheel and shorten the chain, but i usually just find it easier to buy it

the more technical process...

buy a fixed gear hub, if it's the same flange you can reuse your spokes, otherwise you might have to get new ones, relace your rim to that wheel, put on the sprocket, the track hub lock ring, take off the derailers, shorten the chain, it's convenient if your chainrings will unbolt from the cranks, otherwise you might need to find a new crankarm and new chainwheel,

$40 hub
$15 sprocket
$10 lockring
$15 freewheel tool
$15 chain tool
$5 spoke wrench

and then possibly new spokes, and possibly a new crank arm

It's hard to tell you prices exactly, as i've got the convenience of a local shop that lets you use all their tools and work on your bike for free, and he just sells everything really cheap, i got a hub from him for $35, a sprocket for $10, lockring for $4, and i need to get a new crankarm, which i think will only be about $10, so you'll have to check the prices to be sure for yourself, but atleast that gives you an idea on the process,