As a signed-in-user you can create a contact (e.g. register a client) from three places.
Users must be given permission to create contacts. This permission is set using Organisation Preferences.
Steps
Access the Contacts App
Select My Apps in the top menu and select Contacts
Press the Make A Contact button
Select the project that the contact will be added to
Select the Contact Form to use to create the contact
Select the Type of Contact you want to create (Person, Organisation, Object or Animal)
Add the details of the Contact and press Save
You arrive on the Contact Profile page for the newly created contact
To create contacts in bulk from a spreadsheet or other data source, follow the instructions to import contacts.