Do I choose the target or the ally for the Help action?

When I use the Help action, do I choose the monster to distract (and then the next ally to attack gets advantage) or do I choose the ally and the monster (so that only his attack against that target gets advantage)? Or something else?

Help

[…]

Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.