The Install Gentoo wiki from /g/ has a good beginner's guide. Unfortunately, it looks like the wiki is down right now, but here's an archive of the SDR page.

http://archive.is/eJYEy

Just get a $30 RTLSDR kit from Amazon. Plenty of easy/fun projects, only caveat is its read(listen) only. Track planes, listen to TV/Radio/watch your garage door clicker/listen to satellites/the space station/a million other things.

If you like it and want to take things further, buy a limesdr mini or bladerf or something. By then you'll feel constrained by the rtlsdr and know better what you want.

Adalm Pluto if you also want to send.

LimeSDR mini.