The first and perhaps most powerful thing you can do is: pray. Bible Society Australia has some suggested prayers:

• Please pray for the Iraqi Christians forced from their homes in Mosul under threat of death.
• Pray for God’s protecting hand to be with them and his provision for them is plentiful in this time of urgent need.
• Pray that the Bible Society team in Iraq is safe amongst the crisis and are able to carry out their work to reach families in desperate need.
• Pray that others would recognise the plight of their brethren and provide support for them.