How are thre highest moutains formed?
The highest mountains on Earth are formed by "Plate Tectonics":
Two adjacent "plates" in the Earth's crust move towards one another, and by a combination of one plate being forced to move onto the top of the other, and the upper plate tending to fold in doing so, the mountains are formed. http://funadvice.com/r/bve43onev44
The largest mountains on other planets may be formed by other means, e.g. volcanic action, but I gues you are not really talking about them.
The same way normal mountains are made. Over time, rocks fuse together, and ta-daa!! Mountains!
The tallest mountains are just older.
Hope I helped! ;)