Do you want to jailbreak the newest smartphone? Or want to repurpose some neat and cheap childrens' toy into a spectrum analyzer?
Maybe your central heating has a odd habit of keeping your bedroom too hot, and the office too cold even though the thermostat is set correctly?
Reverse engineering by itself is just a tool for building things, because once you figured out how something works, the fun begins when you start tinkering it into something the original designers had not forseen.
So, to cut this philosophical post short: Get yourself a problem you want to solve, then read all you can about it. If it involves reverse-engineering, e.g. analyzing something not yet documented, you'll have to solve that on the way.
And you'll know to ask the specific questions (e.g.: How do I find out the right connection to pins I suspect to be a JTAG port? How can I find out which bits in the serial protocol might stand for what?) when you've come there.