If you've installed WordPress but not an SEO plugin then you're seriously denting your chances of success in the search engine listings. Just simply, the default settings in WordPress do not give you enough control. Currently the best plugin to use is the free one from Yoast.
Once you've installed the plugin, go through each of the tabs in turn and adjust the settings to your liking.
But before you do that, go to the Settings option and change your permalinks structure. I prefer the post name option but if you've got a very big and busy site you could choose one of the other options.
Then move onto the Yoast SEO settings.
The general tab lets you easily link your website with the webmaster tools provided by Google and Bing and also allows you to claim your site in Alexa without having to upload a file to your server.
Once you've done that, you need to move on to the Titles and Metas section.
This is where the real SEO magic happens!
Yoast allows you to force a rewrite of your page and post titles. If the plugin thinks this is necessary for your chosen theme, it will tick the box for you automatically but if you find that some of your page title settings are being ignored then it's worth ticking this box.
What it does is ensures that the title shown in the browser tab and (more importantly) in the search results is the one you chose.
Yoast will give you a preview of your title and will also tell you when you've used too many characters which would mean that Google would truncate it with an ellipsis.
This is not a totally precise science because Google now takes an account of the width of the characters in your title, so it's worth erring on the side of caution and using a few less than the plugin suggests.
There are various other settings on the general tab where you can remove certain types of pages (such as archive pages) from getting indexed and potentially giving you duplicate content issues on your site.
The post types tab is also important.
WordPress typically includes the date of the post before the meta description (the text that shows below your page title in the search results) but the plugin does not allow for this by default in its preview. Instead, you have to tick the appropriate box on the post types page so that the date shows in the preview.
I'd personally missed doing this on my blog until recently which meant that a lot of my page meta descriptions were cut off by Google. Once you've ticked the box - and you only need to do it for the posts option - the preview and suggested number of characters in the description will be accurate again.
There's also an option to change how your default post and page titles look. So if, like me, you do not want your site name in the title, you can remove it from the defaults which saves you forgetting to do it in each individual post.
The other main setting to pay attention to is the one that allows you to link your Facebook, Twitter and Google+ profiles and make sure they're co-ordinated and appear as you'd like them to.