Help making a message box.

I have a post category called “Alert” and want to display the post(s) in a box that I will create with CSS above the main section if a post exists in the category only, like special messages. Can someone tell me the PHP code to do this. I’m thinking if have_posts in Alert, then display this box and text, otherwise the box and text will not show up.

Thanks for any help you can give me.