Assuming your settop box puts out HDMI, you need something called HDMI splitter.
http
/www.amazon.com/ViewHD-Ultra-Powered-Splitter-Model/dp/B00J4D3RTU/
Also be forewarned that because HDMI uses so many conductors (19), each individual wire is very small. Consequently, getting HDMI cables to work beyond about 50 feet is difficult unless you move to much thicker cables. I highly recommend Redmere cables. They borrow a little power from the TV, send it back to the source, and use it to amplify the signal before it's transmitted. This allows for much longer runs without the cable becoming ridiculously thick and difficult to wrap around corners. Their only drawback is you have to plug the right end into the TV...