You cant. HDMI is digital signal, and in general TVs do not output sound over HDMI. If your TV has other audio outputs (headphones, optical), you will need proper amplifier for your subwoofer, with proper input.
Look on the TV and see if there is ...
1) a subwoofer powered analog output from the TV. Many large screens have this. This would be a speaker wire level connection to the subwoofer
2) a sbuwoofer line level output from the TV. Typically an RCA connection. This would go to the line level connection on a powered subwoofer, or to a power amp that drives the subwoofer
If the TV does not have a subwoofer output then you need to get a device that decodes the HDMI signal and gives subwoofer output. A receiver is the easiest way to do this, and they are pretty cheap now. You can run the HDMI into the recevier then out from receiver to TV and to subwooder.