First things first: download your data https://takeout.google.com
Google has created a decent data dashboard for you to drill down into https://myaccount.google.com/dashboard.
Use this to delete the excess data Google has that you're not getting any value from anymore. You can delete all your contacts, photos, groups, calendars, etc. If you rely on some of this, for example, Google Calendar is the only calendar you use. Keep it. Keep the value. Delete the excess.
You can delete your Google+ profile. You probably didn't know you had one.
You can deleted Google Analytics tracking from old websites. Apparently, Google Analytics tracks 25% of the internet. These days we're moving to use browsers that block trackers so why impose trackers upon other people?
You can move all of my files off Google Drive. Make sure you delete the Trash too!
You can delete all of your Gmail, or keep it for archival purposes. You might want to go in and have a spring clean of old emails and folders you no longer need or use. If you've use Gmail email as a login for things or for 2FA authentication you definitely want to hang on to it.
View your Google activity https://myactivity.google.com/myactivity. There's probably a lot!
Delete your Google activity https://myactivity.google.com/delete-activity
If you want: Change Today to All time.
Check what products you have selected. You can do them all at once with All products.
Change your Google activity tracking for the future https://myaccount.google.com/activitycontrols
You'll probably find these are all enabled by default. This is the biggest problem that tech companies need to solve. GDPR should go some way to making this better in May 2018.
You might want to toggle all these to disabled (paused) and unchecked any relevant boxes.
Take a look at even more Google activity options https://myactivity.google.com/more-activity because why wouldn't they all be in the same place?
Check you're happy with everything by doing a Privacy Checkup https://myaccount.google.com/privacycheckup
You might as well do a Security Checkup since you're there https://myaccount.google.com/security-checkup
If you're feeling super brave you could Delete your entire Google Account completely https://myaccount.google.com/deleteaccount
You're done. Have a hug 🤗