Learn about the Unreached people groups and pray for them – see here. (The Joshua Project)
Also, the book, Operation World is very helpful. (Look around at the other resources we have linked to at the Propempo store.
Add Scripture into your prayers and pray that God will raise up laborers for the different fields and people groups – Matthew 9:36-38.
Pray for the gospel to go forth and spread rapidly and that the Word of God would be honored – 2 Thessalonians 3:1-3.
Pray for missionaries and their boldness – Ephesians 6:10-20.