- What does the drop out look like? If it's vertical then you will need a single speed tensioner (or use the existing rear mech as low budget alternative)
- If it's a semi-horizontal dropout then you should be able to adjust the chain tension through it without need for a tensioner
- What kind of hub is on the rear? If it's a free hub then all you need to do is to choose your gearing of choice, add spacers onto it and line it up straight with the front chain ring.
- If its a screw on freewheel then you might need to change out the bottom bracket with a long or shorter spindle to accommodate a straight chain line with the rear wheel
- If you're going to be using the existing crankset/chain ring then you'll need to choose from the three chain rings to run (im assuming it's somewhere in the neighborhood of 53/42/37) and get single speed chain ring bolts to cut the slack