The robots brain has two parts. The microcontroller (Arduino for example) or full microprocessor (Raspberry Pi or desktop computer) or embedded system etc..and the software (Arduino Sketch, C, ROS, MyRobotLab etc)
So the first step is deciding what you need your robot to do now and what you anticipate it needing to do in the near future.
You will look at different board types and the type of software that will do what you want it to do.
Some robot control software allows you to move arms and legs and eyeballs from a nice easy to use graphical interface program and some are operated from the cloud.
Others have no such thing and you must either use some open source code that does these things or modify it or write your own from the very beginning.
ROS is a popular and growing international language to control robots worldwide so this would be a good one to at least know about.
Myrobotlab only runs on certain hardware types at this time in 2021 and is ideal for Arduino.
(under construction Oct 17 2021)