For info on Google Drive storage, see https://support.google.com/drive/answer/6558?hl=en&ref_topic=2375005
In particular, it's important to know that some files don't count towards your storage limits, e.g.:
Google+ Photos: Only photos over 2048x2048 pixels and videos longer than 15 minutes count toward your storage limit.
By default, G+ resizes photos to fit this limit.
Google Docs, Sheets, Slides, Presentations, Drawings, etc. don't count
Files shared with you never count toward your storage on the web
So deleting these files won't help towards your storage limit.
You can sort Drive docs by size, then delete some (to the trash), then permanently delete them.
Gmail attachments may accumulate a lot of storage usage. You can search Gmail for "has:attachment" to find them all, then delete ones you don't need that are large. You can't just remove an attachment from a message or remove a message from a thread. You have to delete the entire thread.
Support site: https://support.google.com/drive/answer/4431192?hl=en&ref_topic=14951