If you are stuck on a deserted island with only WordPress and one extension that extension would without a doubt be the Yoast SEO extension. It will help you optimize your posts and pages for your main keyword. And if you pay even for several keywords. You will still have to go to Google Webmaster Search Analytics to get a broader picture about what keywords Google thinks your page is good for – potential visitors.
The other essential extension
https://wordpress.org/plugins/google-analytics-for-wordpress/ – Monsterinsights Google Analytics, Monster bought it from Yoast.
