Adding to the style.css when making new page template

Hey y’all, I am attempting to create a new page template that i am calling “main-wide” (clever,no?) so I may post the entire width of the page. I figured out how to make the page.php part (got rid of the call to sidebar stuff and changed the id name) so now I need to correctly add the required parameters to my style.css. I notice when I try to add lines to the middle of the script, it seems to throw everything off. (Yes, I’m a crappy at poetry, aka code.) I got that it’s something like:
#main-wide {
blah: blah;
width: 980px;
}
…whatever.
So I don’t know exactly what I’m supposed to substitude “blah” for… please don’t laugh… and how to “add” it to the style sheet, because whenever I try to insert what I think is right, it throws off the footer. I hope this makes sense. Oh, in case you need to know I’m using the Selecta theme. Thanks guys!