Remember, SEO is not just about keywords - but adding and updating content, say by writing keyword rich articles or blog that is regularly updated will help search engine rankings in the future. It will also gradually increase the size of your site and well written copy will attract inwards links. These are all positive factors in improving your ranking.
Articles or blog postings should be around 500 words and not more than 1000 (analysis Tools such as Textalyser help you with word-counts, etc), and should be broken up into about four paragraphs or subheadings with the first paragraph being an introduction with a 'hook' (say, in the form of a question) to draw readers onward into the article. Know the keywords that you are writing against, but in the first draft don't worry too much about placement of the keywords - that way you'll produce readable text that flows nicely. But later you will need to use the keywords exactly how they are used on the site, so the second draft may mean that you have some rewriting to do to get the keywords into position. There are a number of Keyword Density Tools such as Live Keyword Analysis to help you to revise your drafts.
The first paragraph must include the keywords once in the first sentence, and as close as practical to the beginning of that sentence. The first paragraph must include the keyword or words once more. Overall the keyword density shouldn't be more than around 3%.
Each following paragraph should include the keywords once, and the last paragraph must include them, but do not have more than seven entries per posting or you may be penalized by search engines who now look for keyword stuffing. These three or four secondary paragraphs can be structured as a sub-heading containing the keyword with following supporting text. This will allow you to set H-tags on the sub-heading.
Re-read the posting and correct any errors and then meticulously spell check.
Finally, write the headline. The headline is the most important part of the article and it must reflect the contents you have just written plus have good keyword placement: the headline should start with the keyword.