How to add SEO Code like meta, title, on existing pages

I need to update the Meta tags, page title, page descriptions and other important SEO information for my existing pages, and then of course for any newly published pages.
I am using WP 3.5.1 and the original WP coder apparently did not include the SEO data used by the Search engine spiders to catalog my site. Over 59 missing items were found for only 50 pages. I believe after the site was launched WordPress SEO by Yoast was installed. I apologize if this is very basic, but it was left undone and I want to make the changes correctly and completely myself. Thank you.