How do I use my free iCloud 5Gb

It is accessed automatically by anything you sync to iCloud (e.g., iCloud Photos). The options are in Settings > [username] > iCloud > (Contacts, iCloud Photos, Calendars, Notes, Reminders, etc). When those options are on, data is automatically synced to iCloud as changes occur.

Also see: Set up and use iCloud Photos - Apple Support

It is also used by iCloud Backup. See the iCloud section of How to back up your iPhone, iPad, and iPod touch - Apple Support