HDMI cable over 20 meters

I would go to a local computer shop and ask. Chances are that they have the HDMI cable and you may get away with buying a used one and save some money.

The 1.5m HDMI cable is more common in Mauritius and you can get it from most computer/electronic shops at around Rs300. For the length that you're looking, I'd suggest getting it through ebay because it's definitely going to be a lot cheaper than what you're going to pay locally.

If you have HDMI ports on both devices, it's just a matter of connecting the cable from the source to the destination. For example, if you want to connect your laptop to your big screen TV, all you have to do is find the HDMI port of the laptop, plug the cable in and connect the other end of the cable to your TV.

If you're still unclear, let me know exactly what you're trying to connect and I'll give more detailed instructions :)