There are two main ways to sign in to iCloud:
On your Apple device:
Open the Settings app.
If you see your name at the top, tap it. If you don't see your name, tap "Sign in to your [device]."
Enter your Apple ID (or the email address or phone number associated with your iCloud account) and password.
You may be prompted to enter a verification code sent to a trusted device or phone number.
Choose the iCloud features you want to turn on, such as Photos, Contacts, or iCloud Backup.
On the iCloud website:
Go to https://www.icloud.com/.
Enter your Apple ID (or the email address or phone number associated with your iCloud account) and password.
You may be prompted to enter a verification code sent to a trusted device or phone number.
Once you're signed in, you can access your iCloud features, such as Mail, Photos, and Notes.
Here are some additional resources that you may find helpful:
Apple's guide to signing in to iCloud on all your devices: https://support.apple.com/guide/icloud/sign-in-and-use-icloudcom-mm281e3e7d/icloud
Apple's guide to signing in and using iCloud.com: https://support.apple.com/guide/icloud/sign-in-and-use-icloudcom-mm281e3e7d/icloud