After the ability resolves, you control the spell. Any instance of “you” in that spell’s text now refers to you, “an opponent” refers to one of your opponents, and so on. The change of control happens before new targets are chosen, so any targeting restrictions such as “target opponent” or “target creature you control” are now made in reference to you, not the spell’s original controller. You may change those targets to be legal in reference to you, or, if those are the spell’s only targets, the spell will doesn’t resolve for having illegal targets. When the spell resolves, any illegal targets are unaffected by it and you make all decisions the spell’s effect calls for.