Welcome to the Bunnings Workshop community @ruby2020. It's brilliant to have you join us, and many thanks for your question about mixing concrete.
Just so you are aware, it would likely be easiest to use pre-mixed concrete rather than mixing your own. However, if you'd like to mix your own, typically, you'd use three parts blue metal, two parts sand and one part cement. This is called a 3:2:1 mix and is suitable for most concreting applications. So, you'd be using 60kg of blue metal, 40kg of sand and 20kg of cement.