Singlespeed & Fixed Gear - Single Speed MTB

I have an old Trek 800 Singletrack that I'm better off keeping than selling. I want to turn it into a single speed. I want a single speed, winter powerhouse with fenders. Snow tank. I'm not willing to spend a lot of money. For clarification, I won't be relying on this thing everyday during the winter.
Freewheel removal: 5-10
Freewheel (cheapo): 10-15
Chain: 10

So:
Take of extra junk.
Put on a freewheel.
Shorten chain length or get a new one.

Questions:
How do I decide which existing chain ring to use? How do I reinstall it?
Since it has vertical dropouts, do I just then add a chain tensioner as well? If so, should I just use my old rear derailleur? If not, then am I presumably fine with just freewheel, cranks, chain, chain ring?

Thanks,

Max

you can use which ever chainring you please. i would probably use the second. you should be able to take the other two off with and allen wrench, and slip them off by using a crank puller(extractor) to remove your cranks.

you can buy something like this (http://www.jensonusa.com/store/product/CS611B00-Gusset+Single+Speed+Conversion+Kit.aspx) to convert with, but they seem a little sketchy for $20. maybe you can hunt around to find a better one.

finally, you can buy something like this (http://www.jensonusa.com/store/product/RD707Y01-Surly+Singleator.aspx) to keep your chain tensioned.

Step by step instructions (http://www.singlespeedoutlaw.com/issue1/features.shtml#HowTo)

I have a Trek 930 that I just converted to a single speed. I used the middle chainring, and 16t cog, I had to get a new back hub with about 6 spacers in order to have a straight chain line, I bought a chain tensioner as well. I was planning on doing this all myself but ran into problems so I had my LBS hook it up. It cost me about $100 for everything.

Thanks Guys