How about starting from the middle?
😉
Seriously, it's a matter of preference. Never seen anyone start from the top on a normal wall though.
If you're using a guide rod, and/or spacers, you should probably start from the bottom.
If you're not, I'd start from the middle of the wall if I were you. I'd do this because even small errors in the alignment made in the begining, will grow bigger as you get further away from the starting point. By starting at the middle, you effectively halve the chance of any such deviation becoming noticable, atleast to an untrained eye.
Here's a guide on how to acchieve a good distribution of the tiles:
Centering Guide
Use the method described in the post I linked to figure out the sizes of the border pieces if you can't use whole tiles all the way. Mosaics are a bit tricker to do this with though, as the individual tiles tend to be very small. This makes accuratly measuring over large surfaces difficult. It isn't practical to count the individual tiles, and using the the number of sheats can be inaccurate.
A tip would be to place the sheets in a row on the floor (without adhesive of course), measuring the intended surface, and then applying that measurment to the row. That way, you only have to deal with two pieces, not the other 300 or so, when figuring out the distribution.
If you have any additional questions, do ask.
Edit: By the way, be sure to use both adhesive and grout compatible with natural stone. If it's marble, you'll need to use marble-specific adhesive and grout.
