Go to John Bridge Forum This will be your absolute best bet for knowlege. John has a couple books you can buy as well. Top notch how to, others will give you bad advice.
There are many variables when changing a tub to a shower. For one in many municipalities a tub drain is 1.5" in diameter and a shower is required to have 2" diameter drain. Also others mentioned moving the drain to the middle. Also there is proper construction of the shower. And tearing out...