Sample Outline for Talk on Apostasy (General)
For a good, brief history of the Great Apostasy, read "Apostasy" on MormonWiki.com. This is owned by More Good Foundation. You should also study Apostasy on Mormon.org.
1. Tell, in story form, a basic history of the apostasy. Begin by showing it began even while the apostles were still on the earth.
2. When Jesus and apostles were gone, there was no one with the priesthood authority to run the church and answer disputes, so people had to guess at doctrine.
3. Many churches sprang up as people were unable to disagree.
4. Council at Nicaea.
5. Apostasy was foretold by prophets. Insert quote.
6. Restoration--why needed and how it happened. Insert favorite quote on topic.
Note: Be careful not to come across as attacking other churches. Explain it in terms of not having a prophet, and so they did the best they could in many cases, but had no way of getting direct revelation for the entire church. Although your topic might be apostasy, the restoration is the key to the topic.