You can change which fields are visible and required when a user registers with/logs into your site by changing the Profile Properties.

·         Select Manage Profile Properties from the User Accounts drop-down menu

The other thing you'll want to do is:

Go to Admin, User Accounts

From the User Accounts drop-down choose User Settings

There are the options for Require a valid profile for Registration (meaning they'd have to answer those questions, but existing accounts won't) and/or Require a valid profile for login.  Personally, I'd say that if you require a valid profile for Login, you also want to require a valid profile for Registration (it's less annoying to answer those questions when registering as opposed to thinking you're okay and then being forced to answer more questions when you login).