Hi Sam,
I can second Scionate’s suggestion to do some tutorials. But to be more specific to your problem: You said you are already able to control your robot arm with your random float generator, which is great. Now you need to wire your gamepad input to it. Take a look at the Unreal Engine Documentation on Input:
Input | Unreal Engine Documentation
You set up the controller input under Project Settings → Input Bindings.
Then you access your bindings in your Blueprint via an input axis node:
I hope this helps you to make progress.