Converting a MTB to a Road Bike
A mountain bike will never really be a road bike. The geometry and construction of the frame is different. Mountain bike frames are designed for a different posture and are often designed for a suspension fork, as well as generally being beefier.

You can set up a mountain bike with slick tires and drop bars if you want. I've tried this before and the ride quality is a bit off compared to a road bike.

If you want to take it all the way, you'll need to swap out the following:


you may need a rigid fork
you'd need to make sure that your frame and fork can accomodate 700c wheels and is drilled for caliper brakes. Many mountain bike frames will fail on at least one of these counts.
You'd also need to replace your crankset. Mountain bikes generally are geared much lower than road bikes.
your brake levers would need to be swapped to short-pull models meant to be mounted on a drop bar.
you'll need different shift levers probably.


All in all it's probably going to be a lot cheaper to get a used road bike and refurbish it than to convert an entire mountain bike.