As to your question about connecting the Comcast and this to your TV, they both should simply connect to an available HDMI port on your TV, and you use the TV remote to change between the cable box, the Roku, and perhaps over the air TV. HDMI cables are inexpensive and can be found almost anywhere. The HDMI cable carries both the video and audio signal to the TV, so you only need the single connection.
If your TV doesn't have HDMI ports, or it only has one, then you have another issue.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.