Adding a field to content type?

I am trying to set my site up where I can add a custom page.
That's no problem. But now when I Create Content for that page type,
I need to add custom fields. Another description box for example.

But when I go to manage feilds, I try to add a type but the "Type Of Data to Store"
has no choice for me to add a description txt feild.

It only shows: File or Image

How can I add another description feild?

Thanks in advance!