Question

In Pokemon SoulSilver, where do you get Mewtwo?

Answer 1

To get Mewtwo you must Get all 8 Kanto Gym Badges. Then Fly to Cerulean City and go up to route 21. Go to the river to the left and surf down till you reach a cave. Mewtwo is at the end of the Cave and is levl 70 with Psycho cut Amnesia Guard Swap and Power Swap
Source: http://www.serebii.net/heartgoldsoulsilver/legends.shtml

Answer 2

To get Mewtwo in HG/SS, you must do multiple things first.
First, you must get all Kanto Badges.
Next, go to Cerulean Cave, you must go deep into the cave to get to Mewtwo.
If you have any trouble, I've put some maps from Bulbapedia to help you out. (You may have to zoom in, the maps are small.)

1F

2F

3F (Room with Mewtwo.)

Hope this helps!
Source