You have found the right place. I highly highly recommend you start with a pi-zero, which is armv6 based, the armv7 and armv8 on the pi3 is a whole other level of pain. There are other armv6 based pis you can sometimes get the pi1 A+ for lack of a better term.
You will need an sd card of course, dont need a big one a couple of gig is probably the smallest you can buy these days taht will be fine, dont need a full linux image. You will need some sort of serial cable, 3.3v based. sparkfun, adafruit, ebay. an ftdi breakout board for a couple of bucks on ebay plus some jumpers. or one of the cables from sparkfun/adafruit/amazon, etc. https://www.adafruit.com/product/954
be careful to hook it up right.
The first couple of topics in the baremetal forum are pinned and have a list of resources.
baremetal does not mean assembly language btw, normally baremetal is done in C with a small percentage of assembly if needed, usually for things you cant do in C.
But you can certainly learn assembly language on this platform. baremetal or not.
99% of baremetal programming is reading and for lack if a better term hacking, because all documentation is buggy, so you have to test it. The time spent writing the code for your actual application is a small percentage of the overall time. there are so many good examples and resources though that have blazed a trail you can just read those resources...more reading...Each new platform is like a new programming language most of your time is spent not programming your application but instead learning the rules and api calls, in this case learning the peripherals. ARM is not your primary concern, its like english to a book, its the transport that provides letters and words and you need to know that alphabet and language sure, but you are using the arm to get at the peripherals which are broadcom or broadcom purchased ip, not necessarily arm.
Have fun, come back here if you get stuck. Start with a pi-zero. There are other platforms, a plethera of stm32 based microcontrollers you can use to learn baremetal as well for example. The pi is easier to some extent but not documented as well.