Subwoofer amp.(L + R audio to mono)

Hey guys. My old amp from an old computer stereo system died. It contained a 28watt subwoofer. I plan on taking that out, and building my own amp. The subwoofer is mono(has a positive and negative terminal). I have been looking around at various amps, such as the TDA1554, which looks suitable for this application. The only problem with that amp and all the others, is that it requires a L and R audio input and outputs a L and R audio. My woofer only accepts mono. I was wondering if you guys could help me on how to go about this. I hear all this stuff about bridging and how you can combine two channels into one. How does this work? How can i combine my L and R audio into a mono channel? Also one last question. The purpose of a sub woofer is so exaggerate the lower frequencies. Typically 80-200hz i believe. In order to do that, would i just have to add a low pass filter to my amp?

Thanks guys, and hope someone can help me!

Summing amplifier is the method to combine L and R into a mono channel. Total isolation. You can shape the frequency in the same amp by rolling off the highs.