Use gqrx to listen to the radio, public services / air traffic (if legal where you are. Great fun at airshows), ham radio operators etc https://gqrx.dk/download - requires minimal knowledge to get started (select device, put in frequency, mode (AM or FM) and that's it. Just play). Great time:benefit ratio so far.
There's an excellent video series focusing on SDRs at https://greatscottgadgets.com/sdr/
Then use gnuradio to build your own receiver from predefined blocks.
Start with above cheap receive only SDR, but when you want to transmit, buy a HackRF. You do need a ham radio license to transmit. It's not hard (not even highschool level math). Even if you don't do the license, have a look at the materials linked at https://blog.hamstudy.org/links/ , specifically the linked book (free PDF or paid printed). The fundamentals are excellent and it picks you up where most people are (high school math skills, no idea about RF). If you speak German, the best resource is https://www.darc.de/der-club/referate/ajw/darc-online-lehrga... (also available as printed book)
At that point you should have a better idea where to go, and even enough background to read "real" books like http://www.dspguide.com/ (the math is harder of course). DSP is what SDR is all about in the end. But also other radio related books, like the ARRL antenna book.
(There are alternatives to everything of course, but with above recommendations you will not go wrong.)