How do I sign into iCloud to view/ download my photos?

If you have iCloud Photo Library turned on, the photos on the phone are the same as on iCloud, with the possible exception that some of them may be space-saving optimized versions if you have that option turned on. You view them in the Photos app on an iOS device. You can also use a computer (or force a mobile browser to access the desktop site) and login to www.icloud.com and go to the Photos app there. See: iCloud Photo Library - Apple Support

If you didn't have iCloud Photo Library turned on but do have an iCloud backup, you can restore the entire phone (not just the photos) from iCloud but you cannot otherwise view the contents of the backup. iCloud: Restore or set up iOS devices from iCloud