I suggest installing Package Control (http://wbond.net/sublime_packages/package_control/installation
Once package control is installed, you should use it to install zencoding.
The zencoding package gives a large selection of html and css autocomplete options.
Now, to activate autocomplete:
In the lastest beta version, you can activate autocomplete by pressing control+space.
In the lastest dev version (http://www.sublimetext.com/dev
), autocomplete is activated automatically.