Chances are that if you look in File Explorer, the disk name will be shown as “System Reserved”. If this is the case then it’s a small portion of the disk that’s set aside of storing files that are used if you need to do a system restore. In this situation the contents of this drive shouldn’t be changing and none of your data, programs, emails, photos etc will be stored on it - which is why deleting things that are stored on your C drive will make no difference. The warning can safely be ignored.
There is a way to turn off these warnings, but it requires you to use a tool called Regedit, which is a powerful tool that can easily cause you problems if you use it incorrectly, so I’d advise that you just ignore the warnings.
If you do want to do this change, or get someone to do it for you, then here are the instructoons:
https://support.microsoft.com/en-gb/help/555622/how-to-remove-the-low-disk-space-warningIf File Explorer shows the drive as something other than System Reserved, then it could be a removable disk such as an SD card or USB pen drive. In this case you should take a look at what you’re using this drive for and possibly remove it or delete data from it (or move the data to a different location).
Pete.