Is that a picture of the tub you want to tile or are you replacing the tub? Will the area be a tub/shower, or just a soaking tub?
The procedure differs depending on the use.
Greenboard, or moisture resistant drywall, is no longer allowed in a shower. Plus, the paper/coating on it makes it tough for thinset to bond to it.
Hardiebacker is a decent product, but requires some modified techniques to work with. First, it is VERY thirsty, and if you don't wipe it down with a wet sponge prior to applying thinset, it will suck a good portion of the moisture out of the cement prior to it starting to cure. Cement incorporates the water into its chemical structure, and won't achieve design strength (and be a pain to spread or embed a tile) if it is too dry.
If it's a wet area, you have the choice of a moisture barrier behind the material, or one on top of it, but not both. The most common is just a sheet of plastic, attached to the stud prior to the installation of the cbu. The plastic should go over the tiling flange towards the inside, so if any moisture does get there, it can flow into the tub and down the drain rather than behind the tiling flange. The alternative is to use a waterproofing on top of the cbu to keep the moisture going towards the drain. Neither tile nor grout are considered waterproofing...they are a decorative, wear surface not harmed by water.
Note, the Hardibacker instructions will note that you must keep the end of the the material off of the horizontal surface...the bottom edge cannot be where moisture could accumulate, and, you need to use an alkali resistant mesh tape on the seams to help bond the pieces together (you can do this during tile installation to help prevent speed bumps unless you're going to use a topical waterproofing). Hardie is referred to as a fiber-cement board. The fiber in it is listed as cellulose (wood fiber), and if allowed to saturate the board, could cause it to swell. Some surface applied waterproofings are Kerdi membrane, or RedGard liquid and there are others.