There's many ways to get cable channels through Roku. The obvious way is to subscribe to a service like Direct TV Now or YouTube TV. You'll still need an internet connection of course, so in the long run you may not save much money if you cancel your cable service and switch to a streaming service.
If you want free cable channels you'll have to get a family member's password who already pays cable. Then download each individual channel you want and login with the credentials. Not all individual channels have live TV but most do.
If you want free local channels (ABC, NBC etc..) then buy an HD antenna with at least a 50 mile range. They're about $20-$40 depending on brand.
And that's it as far as "TV" goes. Roku offers a lot more like Netflix, Hulu, Amazon as well as a ton of free services that offer movies and shows.