What do missionaries teach?

This depends entirely on the type of missionaries they are and what organization sent them. Many missionaries are church-planting or proselyting, meaning that they were sent by a church or ministry in order to gain converts to that faith organization. These missionaries will teach the doctrines and practices of their sending organization while supporting or building a local congregation. Other missionaries are service missionaries and focus their teaching and service in areas such as health care, education, farming, or building. Many missionaries do both service and preaching.
As an example of what some missionaries might teach, look at the following links:
Living Word Missions Missionary Manual
Hope Mission Trips Evangelism Resources
United Methodist Missionary Training Manual
Evangelical Covenant Church Missions Manual
Preach my Gospel: Manual for Mormon Missionaries
Manual for Episcopal Youth Missions