Who are you?
I am a demonstrator
The biomimetic control software that comes pre-installed on MIRO also happens to be an ideal demonstrator of MIRO's capabilities, and of the class of behaviour that biomimetic control is best at. If your plan is to get MIRO up and running as a demonstration platform, then all you'll need is a MIRO robot and an Android device with Bluetooth and that is capable of installing the MIROapp. See the Demonstrator Manual for details of how to proceed.
I am a developer
Alternatively, you can develop your own software to control MIRO. There are several routes to doing this, but you will need a copy of the MIRO Developer Kit (MDK) in any case.
Typically, you will treat the MIRO robot as a ROS node, and develop your code to run off-board, exchanging data with MIRO through its ROS interface. Example ROS clients written in C/C++ and Python are included with the MDK. This works just the same whether you are using a physical robot or a simulated robot (running in Gazebo). Alternatively, you can develop software to run on-board MIRO, in much the same way, using the ROS interface.
If this is your plan, you need the Developer Manual, though it will be worth working through the Demonstrator Manual first, and getting to know your MIRO a bit (it will help if you have an Android device handy). And before you do any of that, you should look at the Requirements page to check that you are able to provide a suitable environment for MIRO.