If you want to throw a little cash up in the mix you can try the magic gear route. It won't always work with every bike but I've had good luck with my three hardtails. All three are magic capable, one currently is my dedicated rigid SS. One has a rather improper gear in the back though. It worked for slow toolin' around the street but I would spin out way to early to be of any use off road.
Masi Speciale Fixed, Surly 1x1, 2 70's Bianchi folders, Swingbike, Columbia Cruiser 3 spd, Specialized Big Hit and P.2, Cove G-Spot, Xtracycled Bianchi San Jose.
Well, unless you happen to have a frame with horizontal drop outs or track ends you will probably want to look into the magic gears mentioned above or you will need a tensioner. What I would do is go over to Sheldon Brown's site and calculate the gear inches of the ratio that you like, then find a magic gear calculator and find out what can be run without a tensioner, then compare gear inches to see how far off they'll be. If you find that they are too far off for your liking just go ahead and order a tensioner with the rest of your stuff. That takes of picking the gear.
As for what you'll need it really depends on how much of a budget you're on. Really the simplest thing to do would be order a cog and spacer kit (assuming you have a cassette rear wheel) and space it so the chainline lines up with your middle ring up front, then probably throw on a set of single ring crank bolts up front and take off the other two rings. Another thing people sometimes do is just grind the largest ring down some to make a bash guard. Shouldn't need a new hub unless you are just itching to buy one.