To save all your changes you need to commit the snapshots of those changes into your repository each time the project reaches a state in which you want to record. You can find all the functionality related to the commits in the Changes tab of the Version Control panel.
In this panel, you can see the branch, where you are going to commit, along with a list of changed files with the status of the change: added, modified, renamed, deleted. You have the ability to select or deselect all the files using the buttons in the bottom of the panel.
NB: You are not able to switch branches until you commit changes.
This view gives you the abilities to:
In this section:
This also may be useful: