When you click on a barbarian village and select the attack option from the menu, you will be able to select the troops you currently have and how many of them you'd like to send and if you want to send an officer to accompany them.
Each unit has their own number of provisions they require to produce, however most of the units only take one provision -- so sending 10 spears plus enabling the Master of Loot option will complete that part of the quest.
More about battles and how they work can be found on the
Wiki.