I think the best resource to learn from and get started is https://www.sdr-radio.com/

The first $20 USB SDR dongles had an issue with heat causing frequency drift. They have added t-bias which resolved that issue. There is a wide range of SDR dongles available to fit most any budget and level of interest. From about $35 which will get you listening from 25Mh - 1750Mhz. For about $99 you can listen from 1khz - 3Ghz. For about $300 you can listen and transmit from 1khz - 6Ghz. Many HAM radios now days are based on SDR rather than hardware. Which means you can easily add features or hack existing features to do what you want through software. The dongle I went to get started is a bundle from Nooelec that includes an RTL-SDR in aluminum housing, and a small magnet mount antenna base and 3 antenna. It was about $26 when I got mine but they are up to $31.95 now;
https://www.nooelec.com/store/sdr/sdr-receivers/nesdr-smartee.html

Bottom line is that no part of using SDR dongles or getting additional software to work is "easy". If you are looking for "plug-n-play" solutions and are not willing to bang your forehead on your keyboard till you get it to work, this may not be the hobby for you.

First thing you'll need to do is get a USB driver installed. There is a program called Zadig that makes this process fairly painless.
https://github.com/pbatard/libwdi/wiki/Zadig

For listening you'll need to find an SDR program, there are many out there and most of them are free. It seems the two most popular are SDRSharp (aka SDR#, formerly called AirSpy) and SDR Console (V3). I preferred SDRSharp but for some reason could never get it to work quite right. For a benchmark I used a local FM broadcast station since it is a strong 24/7 signal. In SDRSharp I could barely hear it. But in SDR Console (V3) the station is loud and clear, so that's the software I decided to use. Both of these programs have plugins that add features like decoding digital radio, following trunked broadcasts, tracking satellites, tracking aircraft, etc. And again, none of these are "easy" to get to work. But with some cursing and banging your head, you can do it.
https://airspy.com/download/
https://www.sdr-radio.com/Software/Version3

Tracking aircraft actually doesn't use either of the above programs for listening to radio. The first thing you'll need is an ADS-B decoder called RTL1090. https://rtl1090.com/ scroll down and download "rtl1090 - imu". This utility will download everything you need including Zadig. Even though I already had the USB driver installed I did it again just to make sure I had the latest version installed. When it's finished you should have RTL1090 Beta 3 installed. The only thing I had to change to get it to work was the UDP target IP; Click "open" in the upper left. Click Config. Upper left under UDP Target > IP/URL enter "127.0.0.1" and the Port should be "31012". Click Save+close. Click "START". If you start to see some HEX scroll by you are golden. If not you will need to curse and bang your head on the keyboard and keep trying.

The final piece of the puzzle is adsbSCOPE.
http://www.sprut.de/electronic/pic/projekte/adsb/adsb_en.html
Scroll down to "Downloads" and get the latest version, 2.7 as of this writing. It doesn't have an installer so you will need to create a directory and copy the files to it manually. Then go into "install directory\pc_software\adsbscope\27\" right click on "adsbscope27_256.exe" and choose "Send to > Desktop (create shortcut)".

Once adsbSCOPE is installed run it. First thing you'll need to do is go to menu > other > Network > Network setup. At the bottom you will see "presets", click RTL1090. Make sure under RAW-data-client the Portnumber is 31001, and the URL is 127.0.0.1. Click Close.

If RTL1090 is not running, start it. Make sure you see HEX scrolling by in it. In adsbSCOPE choose menu other > Network > RAW-data client Active. You will need to do this every time you run adsbSCOPE. If all went well you should see "Client: connected" and some data start coming in on the right hand side. If not more cursing and banging your head on the keyboard is required.

The map defaults to somewhere in Germany, since that's where these programs were written. Scroll the map to your location. Put your exact location the very center of the map display. Zoom in or out to your preferences. Choose menu Navigation > Set Receiver location. Choose menu File > save default. Choose menu Config > Background Picture > OpenStreetMap. None of the other options worked for me. Choose menu load Maps > Background picture. If you change the zoom level you have to reload the Background picture again.

I hope this helps. Keep in mind I did this in Windows 7 Enterprise 64-bit. If you have Linux, Mac, or Windows 10... I don't know if the above instructions will help. It took me many hours of reading and watching youtube videos, most of which were little to no help at all, to get this thing to work. But now that I have it working I'm really enjoying it. Good luck :>

 

Addendum: I am getting about 80 mile range using a 3 foot indoor antenna. There are tutorials online how to build a dedicated 1090Mhz antenna specifically tuned to receive aircraft transponders. Since I also use my SDR for listening to police, fire, emt, road crews, school bus, taxi, ham radio, FRS radio, etc., I am planning to buy a Discone multi-band outdoor antenna (about $100).

As of this writing the only aircraft required to have transponders are commercial and military (including military drones.) As of January 1st 2020 ALL aircraft flying in commercial airspace will be required to have transponders.