How can I sign in to iCloud?

iCloud is built in to the iPhone. You sign in when you first set it up, and it can be changed in Settings->Account(top of Settings)->Sign Out (bottom of screen).

Once signed in, built in Apps, like Notes, Mail, Files, Contacts etc.. will make use of the iCloud account.

What exactly are you trying to accomplish?

Make sure you are using the same Apple ID for iCloud on both devices.

What exactly are you trying to sync about them?

Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

For Calendar make sure both devices are using the same Apple ID for iCloud, and the same Calendar is selected in the Calendar App on each device.

See here: Create a calendar on iCloud.com - Apple Support