If you're talking about reverse engineering hardware/firmware, then I'd start by picking up a logic analyzer and/or Bus Pirate (something to be able to snoop activity in the circuit). Once you've got one of those, find a circuit to start snooping; figure out how part <a> talks to part <b> and you're well on your way.

If you're more interested in reverse engineering programs, then my suggestion is to pick  any executable on your hard drive (maybe /bin/ls, although it doesn't matter at all) and figure out how it loads, what it loads, and where in memory it loads. Once you understand the ELF header you'll likely have many more specific questions...

Good suggestion with the logic analyzer! thank you :)

So, maybe reverse engineering is the wrong term for what i want to do. What i want to know is what to use to decode "protocols", sniff data traffic, analyze circuits etc. Both software and hardware tools.

Thanks for the replies. I mostly work in linux. I dont really know what to reverse engineer, i just want to get the basics down so I at least know where to start when i find something to break open :)

Depends on what you want to reverse engineer, and what platform you're using.

https://microcorruption.com/ is a MSP430 reverse engineering simulator.

Be prepared to devote a ton of time to it, but it's a great way to get your RISC assembly chops up.