I see. I didn't know HTML templates were being converted to WordPress. So, as of now I've got the following necessary pages: "index page, contact page and blog page."
I'm afraid 3 pages is a little low. How about a gallery page, services page, portfolio page (maybe 3 layouts or smth), history/about page, another homepage layout, another blog layout, regular page, you will need a single blog post page according to the blog page you already got, a fullwidth page, page with left sidebar/right sidebar…
How they should be coded you ask; well we don't know your skills in coding so I guess do your best and you will get feedback anyhow There is no "rule" for a grid system but it will definitely make your life easier, especially when creating page template variations. I also recommend using HTML5 and the elements like