The best way to begin is to read the articles on RTL-SDR.COM and order their RTL-SDR V3 USB Dongle with Dipole Antenna Kit from Amazon for $35.00. Details on their website here, but they prefer US customers to order on Amazon.
You can use any modern computer with a USB port, but much of the open source SDR software is written for Linux. I recommend the use of a Raspberry Pi 4 with a 32GB SD card.
I want to use SDRs in the classroom since they are easy to use and adaptable to many different topics. In the process of developing my lesson plans, I was pleased to discover the lab exercises in Stanford University’s EE 179: Analog and Digital Communications Systems class use the RTL-SDR. While the course content is much more technical than necessary for my target students, the labs (minus the Mathlab parts) can be successful executed by almost anyone with basic computer skills: https://web.stanford.edu/class/ee179/Homework.html
However, if you are interested in understanding the inner workings of signal processing, modulation and encoding, check out the lecture notes: https://web.stanford.edu/class/ee179/Notes.html