error page

does anyone know how i can set up my page so that if the person dosnt choose a size from my drop down menu that it will not let them add it to there cart? i was hoping to have it give an error message that sais "you must choose a size first dummy!"
http://www.ransomapparel.com/store

Under your option name Size in admin > Catalog > Option Values Manager, add an option value of Select or Choose One or whatever you want.
In Attributes Controller, add Select to the product, and set it to "Display Only" and "Default". This will require a real selection to be made.

thanks gjh42 that rocks! so simple i had no idea.
can you tell me how to edit what the error message says? and maybe how it looks?

In general terms...
Do a trial to get the error message, then copy a relevant snippet of it.
In admin > Tools > Developer's Toolkit, paste the snippet into the lower left box, select "all files" and search. This will show you the file and line number where the text is defined, and you can copy that file into your override directory and edit it.

If there is a class/id associated with the message when you view source on it, you can edit that in your stylesheet.