I recently bought a SENTRY ADS-B receiver for the aircraft in order to track the flight.
How could I make Flightradar24 or Flightaware show my aircraft and how to register my aircraft on the website?
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ADS-B is broadcasted by many aircraft and contains flight and aircraft information. This can easily be received and decoded, either by dedicated devices or a simple 20 bucks TV USB dongle. All information is then sent to flightradar and other sites by the members of their community. (there are also other sources for their data)
However, once ADS-B is broadcasted and received by any receiver working for flightradar, the aircraft appears on their map. There is no need to register your self, in fact, you have to opt-out if you prefer to be invisible.
Now, you write about a receiver, and also a short visit of the product page shows that your device is a receiver only. Together with the built-in GPS it might just warn you about other near aircraft (which do broadcast ADS-B). You will need an ADS-B sender to find yourself on flightradar.
In the US, ADS-B Out is required to be installed (by Jan 1, 2020), it cannot be a portable unit. ADS-B In can be a portable unit.
The ADS-B Out market has quite a few options. Some, like the uAvionix, are small and can be installed in place of a winglight or a taillight and include the GPS receiver for position gathering. See skyBeacon, tailBeacon here:
Others, like some of the Garmin products, are intended to be wired in to the existing transponder and use that antenna for sending position information. Garmin also has numerous In/Out products also, including transponders, such as the GTX345, which I just installed to replace my very old King transponder which I wasn't sure actually worked anymore after the plane sat in storage for 12 years.
For FR24, FA and other flight tracking websites to "see" your aircraft, it will need to have a Mode S transponder (or UAT transmitter if in the US) with ADS-B "Out" installed. That is how they track planes. You shouldn't need to "register" your plane with them unless you want to hide it from others. They may not show it by default anyway if you're not on an IFR flight plan, but that's just a viewer setting.
ADS-B "In" isn't required unless you are in an area with poor coverage and want to contribute. How (and if) to set that up would merit a separate question.