Can’t access internet

I’ve recently reformatted a dell dimension 2400. As I try to install AT&T Broadband…it won’t….apparently I Have no I.P. or server….I’m lost….network connections folder is empty as are some others…also….the screen resolution is like 640 X 400 or something another..only, it won’t let me slide it into any higher position… I’d really appreciate any and all feed back..Thank You in advance.