Sarkhan Vol’s first ability affects only creatures you control when that ability resolves.

You may activate the first ability even if you control no creatures. Sarkhan Vol gains a loyalty counter, but the ability will have no effect.

You may target any creature with the second ability, not just a creature an opponent controls. If you target a creature you already control, you’ll gain control of it (which will usually have no visible effect), it’ll untap, and it’ll gain haste until end of turn.