You can practice ON the web service at http://testcover.com/ It’s a live commercial site – So please play nice, and tell me if you find anything. (Subscriptions are free or paid.)
You can practice WITH the service to generate pairwise test designs, because that’s what it does. Briefly, pairwise designs use a small number of test cases to cover all the interactions between test factors. Usually test factors are configuration settings or test inputs. Examples are operating systems, browsers, application options, etc. There’s more info at the site.