Low hanging fruits are usually common errors/mistakes someone made. So for starters I would do the following thing:
Register with Google's webmaster tools.
Crawl your site with xenu's link sleuth (google it, its freeware).
- Look for 404s -> fix them
- Have a look at all titles of you page: are they unique, short and do they have the important keyword in the beginning.
- Look at the depth of your page. Anything above 4 should be looked at.
- See whether all pages send the right status code (404, 200) and the right charset
- Analyze one page with Google's Pagespeed Browser Plugin, fix whatever comes up