Let's see some basic tips for writing the correct HTML expected by the search engines.
Title
<title>A great and unique title</title>
The title content should have 10-70 characters. It shoult include the most important keywords, and each page should have its unique title. Please note that the title of the home page will appear on search results.
Description meta tag
<meta name="description" content="some great description">
The content of the meta tag should be 70-160 characters long. Like the title, it should include your most important keywords, and it should be unique for each page. The meta tag content is also used by the search engines when displaying the results, making them a natural ad for your page.
Headings
<h1></h1><h2></h2><h3></h3><h4></h4><h5></h5>
The page should be properly structured with headings of different weights. It should be only one H1 per page, contaning your most important keywords. Some SEO opinions claim that the keywords on headings are not considered by search engines; in any case, it is a good idea to break the page with headings.
Images
<img alt="Meaningful alternative text" src="meaningful_image.jpg" />
The images should have the alt attribute present, efficiently describing the image. The content of the attribute can be up to 150 characters long. This attribute, and the image's src are important as they are used by the search engines on their image searches.