How do you use the past tense in French?

In French there are three different ways of expressing things which have taken place in the past. It's different to how we do it in English so it can be tricky and confusing to start with. Here I'm just explaining how to use the three forms of the past tense, not how to form them; that's another question entirely!
The first method is what is called the perfect tense or the 'passé composé' in French. This is used to express single actions or events in the past which are now complete. For example:
I went to France - Je suis allé(e) en France
You did this once and now you've finished making the journey to France.
I finished my homework - J'ai fini mes devoirs
You had some homework which you did and now it's finished so it's complete, and you're talking about a single occassion on which you did your homework.
The second method, the impefect tense, or the 'imparfait' in French is used to express repeated actions in the past (this includes the use of 'used to' in English, i.e. 'I used to go to school'; you can't translate this directly into French and must use the imperfect) or to describe things. For example:
We went to France every year - Nous allions en France tous les ans
You went to France every year so you went more than once; therefore it's a repeated action so requires the impefect.
I saw him everyday - Je le voyais tous les jours
You saw him every day so more than once; it therefore requires the imperfect.
When she was young - Quand elle était jeune
My mother was happy - Ma mère était heureuse
These are descriptions so require the imperfect.
You don't really need to worry about the third method that much. The past historic, the 'passé simple' in French, is used in most literary and a few very formal texts. It's not used in modern spoken French so you only need to be able to recognise it in writing; understanding it is very helpful though if you're studying literature as part of your A Level course. Just to show you what some of it looks like, here are a few examples:
The king who lived twice. - Le roi qui vécut deux fois.
They returned home yesterday. - Ils rentrèrent à la maison hier.
It takes time to get it right so keep practising and reading in French!