To define if the password contains non alpah numeric character or not and minumum password length, you can add these lines to the web.config of your project:

<membership defaultProvider="CustomizedProvider">
      <providers>
        <add name="CustomizedProvider" 
             .....
             minRequiredPasswordLength="8"
             minRequiredNonalphanumericCharacters="0" />
      </providers>
</membership>