I have a 10 feet ceiling high wall and want to install 3 glass shelves to put decorative ornaments on them,How high should I install the shelves ?

You can put them wherever you please. In most decorating mags, nothing below 3 ft, to prevent injury from tripping over them and not over 6 ft. so you can reach things without a ladder.

Are these shelves going to be on an empty wall, or behind another piece of furniture? I'd also take into consideration the size of the items themselves, for the spacing between shelves.