It's one of the core helpers, so you'll definitely have it available unless you've got a very exotic setup. From the manual, typically you'll want to do something like:
$styles = 'div#myDiv{margin:10px;padding:10px;}';
$this->headStyle()->appendStyle($styles);
in your view file to initialise your style. In your layout file you'll need to then echo out what you've appended using:
echo $this->headStyle();
Note that both the initialising and that final echo are required.
Edit: this assumes you're doing it inline - if you want to inject a linked CSS file then you'll use the headlink helper; that's the same deal, you initialise it and then echo it out in your layout.