For the home page's canonical URL...
<link rel="canonical" href="http://mysite.com" />
<link rel="canonical" href="http://mysite.com/" />
Where should it be placed within the head section? at the top, somewhere in the middle, or last?
It shouldn't matter where you put that in the head.
The only difference between with and without the slash is that the one without redirects automatically to the one with so if you include the slash you get rid of the redirect. That only applies in actual links though and not in the canonical tag where they are both exactly equal.