Upgrading 8 speed Shimano 105 to 11 Speed Shimano 105
I'll definitely have to change my cassette


Shimano 11 speed road cassettes require wider freehub body. You have several options here:


Get a new rear wheel or hub that supports 11 speed road cassettes.
Get a replacement 11 speed freehub body if there are any (most likely not).
Get a Campy freehub body and Campy 11 speed cassette instead of 5800.
Get this Token cassette. It's an 11 speed road cassette that fits 10s freehub body.



  But what about my crankset? My current chain is 3/32 inches wide and my new chain would have to be narrower so what if it doesn't fit on my crankset? Is that possible?


11 speed chains are narrower on the outer side and same width inside. I'd first try to run older front shifter/derailleur/crankset and see if performance is acceptable. Also note that 5800 front shifter is compatible only with 5800/6800/9000 front derailleurs in case you are going that way.