I agree with Joe.
The damage that can occur from an improperly installed shower is huge and NO you can't just tile over concrete to make a shower base.
A sloped pan must be made which the tile will go on top of...
I would encourage you to look into having a
Kerdi shower installed. Kerdi by Schluter Systems is the ultimate as far as I'm concerned. It allows you to make things in showers such as seats, ledges, and shelves, which previously would have been things that you wanted to avoid as they would be problem areas...
The 2 links above are to the John Bridge website which is a superior information link relating to tile. John Bridge has an E-Book called Tile Your World which goes into great detail about setting tile and the Kerdi Membrane System which I highly recommend reading.
With enough study on the available material at the John Bridge Website and the Forum there you could do it yourself or, at least know very well what you want a pro to do for you and that the pro is doing it right....
Plumbingwise you will probably have a 1 1/2" drain for the tub which will have to be upgraded to a 2" drain for a shower stall... Maybe even bigger if you are going to install a water wasting multi head "Car Wash" style shower, which will also require a water heater upgrade...