For gnu radio or for satellites?
There are lots of web tutorials for SDR using gnu radio, I really like the one Mike Ossman does on the Great Scott Gadgets site[1]. My evolution was one of the cheap digital TV dongles, then a HackRF-1, and at some point next a LimeSDR.
Once you get the SDR side of things under your belt you can start picking specific radio sources to investigate whether it is sun spots, satellites, or something more mundane like the controls for R/C cars, garage doors, or wireless lights.