[+1]: Creatures you control get +1/+1 and gain haste until end of turn.
[−2]: Gain control of target creature until end of turn. Untap that creature. It gains haste until end of turn.
[−6]: Create five 4/4 red Dragon creature tokens with flying.
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.