By default WordPress uses URLs containg a question mark and numbers for your permalinks and archives such as “http://yoursite.com?p=123". In order to increase keyword density of your site for better SEO, You must change that setting to a custom permalink structure.

In your WordPress admin area, click “Options” and then click “Permalinks”.

Select “custom” in the Customize Permalink Structure area and in the box next to “custom” enter this:

/%category%/%postname%

This will create URLs for your permalinks and archives that would look like this:

http://yoursite.com/yourcategory/title-of-your-post

Click “Update Permalink Structure” to save above setting.

Now you are done.