Your free trial will expire seven days from the exact time you signed up (rather than expiring at midnight on the seventh day). Your Settings page will show you this time.
As an example, if you sign up on June 2 at 10:51 a.m. PST, your trial will expire seven days later, on June 9 at 10:51 a.m. PST. The following screenshot shows where you can find this information:
If you want to keep your plan, do nothing. Your plan will automatically activate, and you will be charged at the end of your trial.
If you want to cancel your plan, you may cancel from your Settings page at any time before your trial ends. You'll continue to have access through the end of your trial, and the system will schedule your account to automatically cancel when your trial ends.