Though this topic is not specific for Joomla, it is probably the most important phase in the SEO process. Choosing incorrect keywords will make all your SEO actions worthless.
One of the first things to do with the site is to define the keywords you will be using. You will generally have an idea to start of with, but you should do further research, as the whole site's relevancy will be based on correct use of your keywords.
For some rough checking on possible keywords, I often use a handy tool from the Seorch.eu site. It works very fast with good suggestions from several sources (Google, Bing, Amazon), though it does not provide search volumes.
Therefor and for further fine-tuning, I use the Google Adwords Keyword tool to find similar keywords and aliases. In this example I typed
- joomla SEO
- joomla search engine optimization
In the below example, not all terms seem perfect for my use, but quite often it brings up some very usefull ones.
As you can see, it lists the average number of searches per month, a rough estimation on the level of competition, plus a large list of possible keywords and keyword combinations.
Picking the best keywords from this list will ensure that you capture all relevant traffic for related search terms.
Use your keywords
We will talk a lot about how and where you should use your keywords in the next steps. One option needs to be considered before we dive into Joomla:
If you still need to choose your domain, try to choose one that includes your main keyword(s). Google and other search engines use the domain as one of the most important factors to define the relevancy of a site for a keyword. This site is set-up as Joomla-SEO.net, which ensures that both "Joomla" and "SEO" are always included as search terms, which is much better then a fantasy name. This does not mean that you cannot establish a good SEO ranking with a domain that does not include your keywords, it just makes it a bit harder.
If you are going to use a domain with multiple words, (like Joomla-SEO.net), always use dashes to connect them. Google treats dashes as spaces and can then easily distinguish the various phrases.
Also use your main keywords everywhere possible, from your URL and your main content to your titles, meta-data etc.. But do so wisely, don't over do this (no keyword stuffing). Also this article on magazine.joomla.org has some interesting reading on recent updates from Google to prevent over-optimization.