Want to know how to get started and what are the important factors to look for?

I have been through this type of installation and found the best and cheapest way was to take the tub out and buy a shower stall.

I agree with tickle, premanufactured is way easier than building a custom shower, at least for the pan anyway. Getting started - rip out the tub, tile and drywall. Important factors - Does the drain on your new premanufactured shower pan line up with the tub drain or do you need a plumber to move it? Use cement board everywhere around the shower, do not use drywall, and don't forget a vapor barrier. Think about the door to the shower when sizing it, a standard size unit from Home Depot will be much cheaper than a custom sized one. I'm sure there is more, let's here what others have to say.

Agree with the above post, but you said you wanted a tiled shower.
I just did one of these, about 5-6 x 4-0, and all the materials cost about $1400 at Home Depot.
You can get step by step instructions on tile web sites or you tube