Creating a website with the Starter package is absolutely free, without neither a commitment to purchase a subscription plan at a later date, nor a time limitation.
Should you choose to do so down the road, you will be given the opportunity to add your own domain name (.com, .net, .ca, .us...) or upgrade your website package (more pages, more blocks and tools), by clicking on the button "Change package or options" on the My websites page.
You can find a detailed list of all the additional services on this page: http://www.sitew.com/Website-creation-packages
To upgrade your website to the Premium or Pro package, visit this link: Upgrade my website
You can also test the Premium and Pro packages and all their advanced features for free and with no commitment. To do so, create a new website on the My websites page and choose the free trial.