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:
Drag and drop files from windows explorer to appropriate folder in source control explorer.
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.
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.