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 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 to keep your chain tensioned.