When any person create a new file in the project, he will
add it to the Source Control System in the correspoding to
folder. This process is called "Checkin".

Most of the source control systems provide a windows
explorer like user interface. You can checkin files in
different ways:

1. Drag and drop files from windows explorer to appropriate
folder in source control explorer.
2. Go to appropriate folder in source control system, right
click on the folder name and select 'Add Files'. This will
launch a file browser which will allow you to select files.
3. Integrate with Visual Studio - most source control
systems are integrated with Visual Studio when you install
their client software. This is the easiest way to work with
source control systems. When you right click on any file in
the Solution Explorer in Visual Studio, it will give you
the option to checkin or checkout files.

After you add (checkin) a file to source control , the file
is "controlled" by source control system. If anybody want
to change the file (including the person who created the
file), he has to "checkout" the file from Source control.

When you checkin a file to source control, it will make the
file in your computer 'Read only' so that you cannot edit
it. This is to remind you that the file is controlled by
source control system.

You can add new files to source control, but you cannot
checkin an existing file unless you have checked out that
file.