Upon signing up for a Gmail account, all users are given 15 GB of storage space for their emails and Google Drive files. Once you reach this 15 GB limit, you will no longer be able to receive emails in your inbox, and you will be blocked from storing anything additional onto your Google Drive. You can choose to purchase additional storage space from Google, but the easiest and least costly way to avoid this situation is to simply free up storage space.
This article will guide you through some options that can help.
Before beginning, it may be helpful for you to see a breakdown of how your storage space is being used. Google offers you two ways to do this: the first being the Storage website, where you can find an overview of your account in both percentage and chart form, and the second being the Manage option found at the bottom of your Gmail page. Simply hover over the Manage button located next to the text reading, "X GB (Y%) of Z GB used."
If you'd like to pinpoint which files are taking up the most space in your Drive, head to Google Drive, and click the All items options in the sidebar. Then, click Sort > Quota used.
Google Drive will re-organize your files, placing the heaviest ones on top of the list. This will allow you to easily delete the biggest files that you don't want.
Any messages that are automatically marked as spam are placed into Gmail's Spam folder. Depending on the number of emails received per month, this folder can easily become very bulky. Do your best to delete files from this folder.
Any deleted files will continue to take up storage space until you empty your trash. Google does this every month by default, but you can free up space more immediately by just deleting the files.