Militia Rallier can't attack alone.
Whenever Militia Rallier attacks, untap target creature.
All attackers are chosen at once. You can't attack with Militia Rallier, untap a tapped creature, then attack with that creature.
Untapping an attacking creature doesn't remove it from combat.
You may target an untapped creature with this ability. For example, if all of your creatures have vigilance, you aren't forced to untap an opponent's creature.