There actually isn't a great transition option between a proprietary product and something like Wordpress. You would need to take all of your personal content from the site builder and save that personal content to your local machine. Once that is done, you would be able to purchase a web hosting account and install wordpress. After installing wordpress you would log into the dashboard of wordpress and upload that personal content from your personal computer back up to the wordpress then design your new wordpress website.
The backend will require you to change the domain name DNS from the site builder over to the new ip address you will get from the web hosting account.
Tip: DNS propagation is 1-48 hours, make sure to give the DNS enough time to propagate.