Bonjour Helena,
That's a very interesting question indeed!
Let's point out first that we're talking about the relative pronouns here, which are used to introduce a new information in a sentence (as opposed to question words like "what?").
Once you know this, the separation is quite simple: "qui/que" can mean "who/which/that ..." whereas "ce qui/ce que" mean "what ...".
La fille que je regarde. (The girl whom[m]/that I'm watching.)
Je fais ce que je veux. (I do what I want.)
Je fais ce que je veux. (I do what I want.)
I hope that's helpful!