The users can choose to Sign up for a business account (@domain.com) or personal (@zoho.com) account by choosing the relevant option from the Zoho Mail Pricing page.
The Super Admin users, signing up on behalf of their Organizations, have to provide their domain details to create the Organization account. The next step will be to choose the username to complete the process.
The Sign up page is similar to the one below. You need to provide username, alternate email address, password to set up an account with Zoho.

The alternate email address is mandatory to confirm your account with Zoho. If the account is in unconfirmed state, the user may not be able to send emails. The unconfirmed account will not have the complete privileges in Zoho account.
You will receive a confirmation email with an activation link with the process to activate your account. When you click on the link, you need to provide the same username and password provided at the time of sign up for the account.
The personal users of Zoho Mail sign up for the free personal account to get the @zoho.com email account. The users would have chosen a user name at the time of registration.
In the case of business users with the custom email address @yourdomain.com, the super admin will be creating the email accounts for the users of the Organization. Hence the users will be asked to choose a unique username, when they sign in for the first time. The users need to provide their email address and the password given by the admin when they sign in for the first time.
The user name chosen will be unique across all the zoho services and is mandatory for accessing the Zoho accounts.
Zoho Mail allows you to utilize your Google/ Yahoo/ Facebook credentials, to access Zoho Services using federated Sign In. You would be asked to choose a Zoho username, when you use a federated Sign In. When you use federated Sign In, you will be able to access only the webmail for accessing your email account.
In case you want to access your account via POP/ IMAP or Active Sync or want to enable Email Forwarding for your Zoho account, you need to generate a Zoho Password for the account. Follow the steps below to generate your Zoho Password.
Steps to generate password for Federated Sign In Users:
Zoho supports SSO and all the Zoho Applications share the same credentials and the same profile settings across the services. Hence all the settings can be accessed and changed from https://accounts.zoho.com/

Manage your identity and account credentials in Profile section. The following changes can be made here.
The users can change their Password from the General section in Zoho Mail Suite.
Steps to change the current Password
In case the emails are displayed with incorrect or irrelevant times that do not match your current time, it is possible that the Timezone chosen is incorrect for your account. Follow the steps below to choose the correct timezone.
Steps to choose the correct Timezone