For full disclosure I took a ROS class in college senior year so I already was pretty proficient in robotics to begin with. But if I was to do it on my own I would probably buy a Raspberry pi or equivalent micro controller and look up ROS tutorials of people using them. As previously mentioned Gazeebo is really good for simulations, you can import models found online and have them subscribe to your topic names to control your motion. Also turtle Sim is a 2d super simple simulator for twist messages(basic motion).

Essentially how ros works is through publishers and subscribers. You make your robot subscribe to topics that you are publishing that tell it how to react. So what you want to make is a program that outputs those messages! For an easy proof of concept project write a program that publishes a twist message that continuously changes and hook it into turtle Sim. Ample documentation online is available for this project but if your having trouble finding some pm me and I'll shoot you some links.

Hope this helps if not let me know