To login to your WordPress site and start editing content, visit your website, and in the web address bar after the website address, visit /wp-admin
For example, if your website is currently at the domain example.com, then you would visit example.com/wp-admin
Here you will need to login with your username and password that will have been provided to you. Now your in the backend of WP. You can click on the website's name to visit the front end of the site. Clicking that link from the front end will take you to the back end.
You will notice a black bar at the top when viewing the website, which gives you some editing capabilities.