So it's a little weird, but they're not technically Arduinos. Arduino makes many microcontrollers and related things, but SparkFun is actually the company that makes the Pro Micro that is generally used for these guitars and other controllers and keyboards and things. It's based on the Arduino Leonardo, but it's open-source so there are many clones and they're much cheaper. I've had success with: AITRIP, KeeYees, HiLetgo

I believe the Arduino starter kits usually come with an Uno, which will technically work, but it's more difficult and I really don't recommend it.

Apart from that you'll need: a soldering iron (controllable temperature is important, and basically every iron will come with some solder), small insulated wire (you can salvage this from inside old cables), screwdrivers (most likely a large and a small Phillips for that guitar), a micro-USB cable, and a set of helping hands.

While not necessary, I recommend: a multimeter, wire like this that you can strip with just your fingernails, tweezers, side cutters, a hot glue gun, parts trays to keep things separated, and a fan that will pull solder smoke away from you.