Your standard Gmail / Google account has 15gig of free available storage space, this space is taken up by Google Drive, emails, photos, documents and backups.
How much space have you actually got available? To find this out, go to one.google.com and check out the Storage box.
Storage: In this example 65% of the google storage is used
Click "View storage details" to see where your space is being used
Clean up space: In this case shows the possibility of cleaning 9gig
To clean up your storage space by emptying your Deleted emails, Spam emails and Google drive Deleted files, click "Clean up space"
Here you can review and clean up your storage
Clicking the review link in each of the above will take you to your Deleted emails, Spam emails and Google drive Deleted files.
Select All or Select the individual items you wish to Delete Permanently
Always keep in mind your Deleted folders are a safety net to catch accidentally deleted items, it is NOT for storage.
Below Discarded items you also have Large items, from here you can select things you wish to remove from your Google storage. Keep in mind that files removed from here cannot be recovered from the Trash.
You may wish to download anything from your large items section to your computer or other storage media before deleting.