Well here's a simple guide to do just that!
First of all, you'll need to log into your WordPress Dashboard.
Once this has been achieved, scroll down your Dashboard until you see the "Settings" tab, then hover over it, and click "Permalinks"
Now you're on the Permalinks tab, simply mark the "Post name".
All that left to do is hit "Save".
Now when you visit your site and click links, you will see links like /my-new-page/ instead of the ?postid=2