SEO or search engine optimizations are website elements and activities to emphasize some content of the site. This important content are keywords that you are trying to rank for. The first step of SEO is selecting keywords or search phrases that have low competition and high search volume. The next step is modifying web pages to contain relevant keywords in elements like page title, meta description, url, header tags and other important places. The final SEO action is promotion of the website. This is done mainly through various link building strategies. Every backlink to your website contributes to its popularity and authority. But the most important on-page SEO element is the domain.
Domain is the name of the website as it is seen on the internet. Therefore, it should be either short easy to remember word or keyword related to the content of the web pages. If your website is build around one keyword or search phrase, the best approach is to use this keyword also for the domain. This means trying to register MyKeyword.com on the first place and if this name is already registered to try alternative names. Having the main keyword in the domain name has two benefits. Such domains clearly describe your website without visiting it and consequently the domain is SEO friendly meaning that it explains the content of the site also to search engines.
If the target keyword is not available for the com top level domain you can try with org, net or info domains. There is no significant difference among them–they are all general purpose domains with no specific geographic target area. If you can still not find a free name you need to slightly modify your keyword by adding a word that makes sense in the context of the site. For example MyKeywordReviews.com, MyKeywordInfo.com or MyKeywordShop.com are typical solutions where the main word is already registered. Such variations have still your main keyword in the domain and are still considered SEO friendly.
With proper additional word it is almost always possible to find a free domain name. But in some cases it is also possible to use only the keyword as domain name and selecting one of the country top level domains. If you are targeting only one country or only one geographic area then this approach is equally effective as using general top level domains.
In every case you should choose domain that is somehow related to the website. If it contains the main keyword it will be easily remembered and SEO friendly.