I think if you're just starting you should head right to Google. You'll find tons of getting started tutorials.
Do you mean for wc3 validation or in general for on-page optimization? for optimization concentrate on meta titles, descriptions, alt tags, h1/h2 headings etc.
What do yo want to change in source code.... means you want to do on-page optimization or simple change the layout of website....?
Hello, You can ensure if a site is already running Google Analytics by inspection the site's source code applying your browser, and finding for the Google Analytics following code in your page's source
Find a HTML tutorial and then PHP tutorial. It will take time to learn. I built my own content management systems. SEO Friendly.