I'd start with this
10 Things to do before the Spiders come: most of them can be done relatively quickly.
Be sure to run your site through
SiteReportCard: Web Site Optimization and Promotion Tools to check for broken links, and other problems.
Then I'd get a sitemap made,
Create your Google Sitemap Online - XML Sitemaps Generator put it on your website, with a reference to it in your robots.txt