I have just finished watching the Shannara Chronicles TV series and in the title sequence it shows that Trolls and Gnomes are descended from Humans. However Elves are not on the family tree anywhere. I've looked at some of the other answers and in the books they say the Elves were hiding during the time of man, but where did they come from in the first place? Are humans and Elves related or are they from another planet or dimension?
They were always there, since the dawn of time.
The books do not give the answer on particular details on the genesis of the Shannara elves. Unlike other races, they were always there, as were other faerie creatures created by the Word (somewhat similar to the Middle-Earth Elves, who were firstborn, i.e. before the ascent of the race of Men).
In Genesis Of Shannara, it is revealed that the Elves had contact with the Men, before the men became aggressive and started developping advanced weaponry and war machines, which lead them to their doom, called The Great Wars. Elves then became secretive and kept to their closed community of Cintra. Their magic allowed them to counter pollution caused by Men (industry, later - nuclear fallout).
Note(also FWP disclaimer): The answer is given according to the books. The TV series may differ and in due course, offer an alternative version of Elves' genesis.