Wireshark, plus a book about using Wireshark. I have "Wireshark Network Analysis", which looks to be shockingly expensive right now - to be fair, it's worth whatever price, but maybe not while you're speculativey getting your toes wet - and there are others out there. I am nowhere near an expert (I've only worked through about 1/3 of that book - just enough to solve the problem I had at the time!), but that's what's taught me what I know, and I know can take me further.