Lightning Storm has an activated ability that can be activated only while the spell is on the stack. Any player may activate that ability. When the ability is activated, it goes on the stack on top of Lightning Storm. When the ability resolves, the Lightning Storm spell is affected.

When you cast Lightning Storm, you immediately have a chance to activate its ability since you'll have priority. If you don't, and all other players pass, the spell will resolve. You won't receive priority again -- and thus won't get another chance to activate its ability -- unless someone else activates Lightning Storm's activated ability, casts some other spell, or activates some other ability.

After an instance of Lightning Storm's activated ability resolves, the active player (not Lightning Storm's controller) receives priority.

When Lightning Storm's activated ability resolves, if there are no other legal targets to choose for the Lightning Storm spell, you must leave the target the same, even if it's now an illegal target.

When Lightning Storm leaves the stack, its charge counters go away.