WordPress have full instructions for setting up your blog with SVN.  I’d recommend sticking to the ‘Tracking Stable Version’ section and controlling your upgrades rather than checking out the trunk and risking an unstable install.  This basically comes down to a single command to be run within your wwwroot.

svn co http://svn.automattic.com/wordpress/tags/2.6.2 .

When a new version is released, you can switch to that with:

svn sw http://svn.automattic.com/wordpress/tags/2.6.3

What they don’t mention, is you can do exactly the same with your plugins using the repository here…

• http://plugins.svn.wordpress.org/

For example, find the plugin you want, and checkout to your plugin folder:

svn co http://plugins.svn.wordpress.org/share-this/tags/2.3/ share-this

As before, you can switch to new releases (make sure you’re in that plugin directory first) with:

svn sw http://plugins.svn.wordpress.org/share-this/tags/2.3/

And should you forget the URL you’ve got it from (like when you see there’s an upgrade available, running ‘svn status’ in that directory should help you out.

Once you get the hang of it, it really saves so much more time than that slow download, unzip, copy process you’re probably all used to, and you stand a much better chance at maintaining your own modifications without rewriting them all the time