Building a fixed is simple.
Have a search on here, ive built a few up on the cheap. Never as good as my expensive one though.
The geared chain you will have already wont be good, get a new one. I say go for 1/8th, but it doesnt really matter too much if you go 1/8 or 3/32.
No, you cant take a sprocket off the back and use that, dont be silly
WTF is a rear chainring?
Do you have horizontal drop outs for the back wheel or verticle?
Right, ill do through this.
To make a simple, cheap FIXED here is what you can do. It wont be the best, but it will work.
Take off the gears, if you have a screw block(gears) then your laughing.
Get a fixed sprocket(fixed sprocket just screw on) and you sort of have a fixed back wheel.
You then take off the small chainring on the front(or leave it on) and put the chain on either the small one if you have 3/32 sprocket, or if it looks like you can get the chainline right, the outside one.
To get the chainline right, put the chain on, stand at the front of the bike and use the top tube as a straight edge. The chain should run paralell to this, without a bend in.
If its right, then tighten up that sprocket with a chainwhip. You want it on just right, pretty tight, or it will loosen itself off. You can put locktite on the threads to keep the sprocket on if you like, or a BB lockring on to keep the sprocket in place.
You may need to put a BB lockring or spacer on before the sprocket to get your chainline right.
****SOME CALL THIS THE SUICIDE METHOD BECAUSE THE SPROCKET CAN LOOSEN OFF, SO ITS UP TO YOU IF YOU DO IT, BUT I KNOW PEOPLE WHO NEVER HAD A LOCKRING AND NEVER HAD THE SPROCKET JUST COME OFF*****
Or, the decent and proper way is to get a fixed crankset, well worth the money i find. Ive gone through 3 cheap ones when i had built up cheap fixeds.
Good strong back wheel, i had one built when i first went fixed and its still going strong, had new bearings but thats it.
You will probably need to respace the backwheel though, whats the spacing on the back wheel of your frame? 120mm is the norm, and this is what you get hubs in. But depending what hub you get, if theres enough axle left over, you can put some spacers on, and bend the frame in abit(if its steel, think your frame is?)
You can buy fixed hubs which are wider, but these are less rare and i think more expensive? Company down in London do them, and you bolt the sprocket on, not screw it on.
Bottom bracket wise, you may probably need one thats narrower. How i did it was take the bike to my LBS, and he tried a few to get my chainline right.
If your going to paint it, then you first need to strip the paint down. I had my first fixed bike painted by a brothers mate.
Strip it with nitromorse(be careful with this, very very strong!) and then give it a good wash.
You then fiddle to use some fine sandpiper and lightly key the surface(think you can get special pads that do this cant you?) then give it a good wipe with a wet cloth.
Then you can start your spraying. No idea how though, should be on the can, i just stripped my frame down and my brothers mate painted it.
I have built a few up now, and they are simple. Alot easier to work on. You do just have to understand how they work. Which isnt hard really.
If your in a cycle club, then thats good for help, alot of the older guys and girls rode them in winter, so some will have parts to give you/selll you cheap and also advice to give/help to do it.
Please dont call it a fixie or i will shoot you. Makes it sound stupid.
what will i need to do to prepare the frame and then what about the coat ontop of the colour to protect the paint from chips, anyone recommen some good paint? 
