How do I know whether or not I should use the subjunctive mood after "que"?

The subjunctive mood is by definition a way of speaking that shows the speaker's personal attitude towards what they're saying. I'm sure that the textbook you're using explains all the basic situations when you need to use it (expressing order, wish, obligation, emotions etc.). That's why I would rather just present you an easy way of remembering which verbs are followed by the subjunctive mood and which aren't. Whenever you come across a "tricky" sentence like, for instance, "je crois que..." stop for a second and think - would I classify it as a "verb of the head" or a "verb of the heart"? You'd obviously say it's a "verb of the head" as that's the part of the body we use for thinking ("je crois" = "I think"). Now you need to remember that after such verbs we always use the INDICATIVE - "je crois qu'il fait beau aujourd'hui". Let's compare it with a "verb of the heart" now. "J'ai peur que..." would be followed by the subjunctive mood (e.g. "j'ai peur qu'il revienne ici") because we say emotions - such as fear - come from the heart (with the most important exception - for which we all love French - being the expressions "je sens que..." which is followed by the indicative). Let me know if you need more clarification or examples!