Measure the shower
A standard alcove has a back wall the length of the tub or pan (usually 60") and two side walls (usually 30"–36" deep). Tile at least to the height of the shower head. Most people go to 84"–96", or all the way to the ceiling, which looks cleaner and avoids a trim line.
How the layout is planned
Columns are centered on each wall, choosing a grout joint or a tile on the center line — whichever leaves the wider cut pieces in the corners. Full rows start at the bottom and the cut row ends up at the top, where it's least visible. The planner warns you if a corner piece would be a thin sliver.
Shower niche
A niche between studs at 16" on center can be up to about 14½" wide without changing the framing. Set the bottom at a comfortable height — 42"–48" is common — and slope the bottom shelf slightly toward the shower so water drains out. The niche adds its back plus four 3½"-deep sides to the tile count, and needs edge trim or mitered tile on the front.
Shower floor
Small mosaic tile (2" x 2" or smaller) follows the slope to the drain and gives more grip because of the extra grout lines. The floor is figured in 12" x 12" sheets with 15% extra for cuts around the drain.
Waterproofing comes first
Tile and grout are not waterproof. Behind them you need either cement backer board covered with a liquid or sheet membrane, or a foam board system. Choose a membrane rated ANSI A118.10, and lap it onto the shower pan. The planner covers all wet walls and the floor, with 10% extra for overlaps.
Thinset, grout and corners
Use polymer-modified thinset on walls, and a mortar rated for large-format tile when any side is 15" or longer. Large tiles also need a deeper trowel and should be offset no more than a third of their length. Use unsanded or epoxy grout for joints of 1/8" or narrower. In the inside corners and where the walls meet the floor or tub, use 100% silicone instead of grout — those joints move, and grout there cracks.
Doing a kitchen wall too? Try the backsplash planner, or check the bags with the thinset and grout calculators.