Every article needs a strong introduction to keep the reader on the page, but the first 100-200 words are also important in terms of adding SEO value to a page. Your article on preventing cancer in dogs should use key words, and variations of those key, words as roughly 5% of the introductory words. Search engines often scan the first part of a page more thoroughly and want to be sure the title, body, and tags, all match.
Your entire article should be at least 300 words or more. Many search engines do not even look at shorter articles since they are assumed to be of low value, lacking sufficient information and data to be useful to a reader.
In the case of writing about preventing cancer in dogs it should be fairly easy to write an article that is at least 300 words. You will want to use your key words throughout the page although they may not be as heavy as in the introduction.
Most often when you write and article, or blog, you are requested to add tags. You should always have at least 6 tags, and may find that adding more than 15 is pointless. Some search engines place more value on tags than other features, but tags are easy to fake to get views as a result of deception. As such many search engines also check to be sure the article matches the tags. False tags would be tagging your article with popular search terms, such as celebrity names, rather than tags related to the subject of your article.
Consider different variations of each tag word. In the case of writing in regards to prevention of cancer in dogs, some good tags might be: Cancer, dog, dogs, preventing, prevention, prevent, how to, pet health, tumor, tumors.
Search engines also look to see if a page has outgoing links to pages with good ratings, or pages with bad rankings. If a page has many poor quality outgoing links, links to low quality sites, it will not rate high in terms of SEO. If a page has good quality outgoing links, links to trusted sites, it will rate higher for SEO.
Incoming links are created when other sites link to your page. If somebody writes an article on dog health, for example, and they link to your page that helps your SEO. You can also link your own related articles to each other to help boost your SEO. The more incoming links a site has the more it helps your SEO.
Overall a better quality page should naturally have more SEO than a poor quality one. Search engines want to give the searchers exactly what they are looking for, as such they have a formula for finding what they think are quality articles, all you have to do is write.