Where to start tiling is very important so spend some time planning the layout of the tiles before you begin.
There is no definitive way to do this and it may well depend on your particular circumstances like positions of windows, walls and alcoves,etc.

A lot of walls in houses will not be parallel or square. The vertical corners may not be a perfect right angle with the horizontal. So if you start from one side or the very bottom your tiling may not end up square and not look good. By the time you get from one side of the room to the other the last tiles may be noticeably misaligned. With this in mind the way I like to do things is;

At a point low down, approx. ¾ the height of a tile from the bottom make a true horizontal using a laser level (or spirit level). Lightly nail a timber batten to the wall.This will be your start point and all other tiles will line with this

Measure the mid-point between each corner.The aim is to start tiling in the middle along your timber batten and end at each side with a part tile. Try laying out your tiles on the batten to measure out the positioning, or you could mark the position on the batten. When you're happy you can apply the adhesive and lay the first tile. Once you've got the first row of tiles on the rest should follow nicely. Use tile spacers to get each tile line up perfectly.

Where to start tiling in kitchens

Above a work top you might just want to have several rows of tiles. Usually the work top will be fairly level so you can simply start with your first layer resting on the work top. If you are tiling between two corners start in the middle with whole tiles and work to the corners ending in part tiles
A couple of examples are shown below.

Tiling above a work top
in a recess

P= part tile

w=whole tile

A simple L-shaped layout above a work top

p=part tile

Tiling round a window recess
Again there are several ways to do this but remember whichever method you choose that if you are tiling elsewhere they line up with each other.

Using this method you will probably have to do some tiling downwards. Use masking tape to hold tiles in place with higher ones which you've let dry first.

Edging strip

The edges of some tiles are not made to be seen and so on outer corners they will look better with edging strip. This simply fits behind the tile and is held in place by the tile and adhesive.

Where to start tiling floors
Tiling floors is fairly straightforward (but see section on preparing surfaces)
Again, like tiling walls, because edges may not be square DON'T start at the edges. Mark a line running parallel to a wall so that your first row of tiles runs right down the middle of the room. Layout some tile so that you can ensure you end with roughly equal part tiles at each end. (Depending on your room and tile size you might find you need to start with a row of tiles on either side of that centre line so as to end with a decent sized part tile at both edges.

Work out from the middle ensuring all full tiles are perfectly lined up and square.

Plan view(from above) of floor tiling layout

p=part tile