We've packaged our Git (and GitHub) lessons as 'Learning Trail' which is divided into several 'tours'. Each tour consists of a series of lessons and covers the Git knowledge required for a specific usage.
Git Learning Tour
→ Tour 1: Recording the History of a Folder
→ Tour 2: Backing up a Repo on the Cloud
→ Tour 3: Working Off a Remote Repo
→ Tour 4: Using the Revision History of a Repo
→ Tour 5: Fine-Tuning the Revision History
→ Tour 7: Keeping Branches in Sync
→ Tour 8: Working with Remote Branches