Got a new monitor for my computer?

My old monitor died so I purchased a new one. It's a thin screen and wide. My old monitor was, well, like the old ones - with a huge back to it and not very big square screen. I can't figure out how to fix this new monitor so the screen doesn't look stretched out. As I type, this page looks squished out. I've gone through the manual but no luck there. I also tried the cd it came with, again no luck. Is it that my computer is too old to be able to use this new kind of monitor? Thanks for any and all info!