I do not understand when or when not to use the subjunctive in Spanish.

The Subjunctive in Spanish is not so much a tense, but rather a mood. It can be used to express subtleties in language that emphasise desires, wishes, hypotheses and uncertainty. 

In general, I would employ the subjunctive to express a statement with doubt, or indeed a value judgement that is not fact. A person's desires and wishes are not set in stone, but rather opinion. They are subjective statements that you make, based on your own experience. For example, when I say "I hope you are well - Espero que estés bien", I do not know exactly if you are well. It is my desire that you are not sick. 

Contrastingly, I would not use the subjunctive to say "There are 10 million people living in London - Hay 10 millones de personas viviendo en Londres". This is a fact!

Similarly, when you say "it is normal that they are rich - es normal que sean ricos", it is in my opinion that they are rich. For me, it is indeed "normal" that they should be wealthy people. 

There are a large number of gramatical constructions that oblige the speaker to use the subjunctive, for example the connector phrase "para que" (so that). These must be learned by heart but some of the most common ones are included here: 

para que + subjunctive (so that)

menos que + subjunctive (unless)

a pesar de + subjunctive (despite)

As you can see, these connector phrases mostly imply a value judgement to follow. The majority of cases in which you will use the subjunctive are subjective. To help you to remember, I would always remember the saying "the subjunctive is subjective" or "el subjuntivo es sujetivo" to help you remember when to use or when not to use it! 

There are some more less common cases too, but as we progress I will flag these to you very clearly. I hope that helps you somewhat.