How do you get the keyblade from sephiroth in Kingdom Hearts 2?

You don't get it from him exactly, but you have to beat him in the first shot I believe. This activates a cutscene.

In the cutscene, it shows Sephiroth asking Sora if he passed on his message to Cloud. Cloud shows up and they begin to fight. Tifa, who is a possible choice for his love interest in the Final Fantasy world, shows up and gives Cloud her light. He accepts it (you can tell because he starts glowing) and Sephiroth and Cloud vanish. Tifa sighs, and goes to search for him again. Before she leaves, she gives Sora the Fenrir blade (named after Cloud's motorbike).

Hope I Helped!