When you signup for a new account, you are then sent an account email usually titled "Your Hosting Account Is Ready! (Account Information)" Emails can also be viewed online in our client area.
In this email, you will find the section "cPanel Login Details" which give you the details to login directly to your cPanel which can be done by adding "/cpanel" after your domain.
Once in cPanel, you can change this password by going to "Password & Security" in the bottom.