If you have never used this new Web-Site before then you will not have an existing Username and Password, even if you are an existing Member. You will need to register via "Create new account" with a Username of your own choosing (that hasn't already been taken) and Password, during the registration process you will be asked if you wish to link to your existing Society Membership.
If you have definitely registered on this new Web-Site previously, but have forgotten your password, then click on "Request new password" and use your existing username or email address when filling in the form, you will then automatically be sent a new password.