By default, you get a 10GB windows profile.
Delete unwanted items from your desktop/documents/downloads folder and empty the recycling bin to free up some space.
If you alter the cached mode in Outlook or sync your full One Drive, this could take up more of your allocated memory. Lower the cached mode.
If you need to purchase more space, please speak to your account manager. Alternatively, please raise a new case online and reference the title of this article.