Start with the classroom_activities.
This is an in-progress collection of tutorials about the foundations of Software Defined Radios. Readers are encouraged to supplement with other resources.
- RTL-SDR -- Top recommendation due to low cost; receive only
- Tiny Spectrum Analyzer -- Second lowest cost (around $90); built-in screen; can transmit and receive. From what we can tell, it doesn't have full SDR capabilities, but it's great for scanning.
- PlutoSDR -- Second recommendation due to full-duplex ability
- HackRF One -- half-duplex
- Nooelec NESDR SMArt: receive only