- styles
Please take the time to write new css for the divs. Tables are ancient and break easily and are a total nightmare on mobile devices.
- overrides
If you do 1. you won't need them. Anyhow they're really easy.
A template override is simply a component or module view template which is copied under the your template's html subfolder (in a slightly different folder structure) and Joomla will use those instead of the original files.
So for example if you want to override article view, which is in
/components/com_content/views/article/tmpl/
its override would be placed in
/templates/your_template/html/com_content/article
One small catch: Make sure you always copy the whole tmpl folder of components, you must override all layouts inside the template.
You can find more info on http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core