When would you use the passé composé, and when the imparfait?

One important thing to bear in mind is the time frame of your action. If are writing, for example, "last year, I went to France", there is no doubt it would be 'l'année dernière, je suis allé(e) en France". The action is complete, as the trip to France was a one-time event, and now complete, therefore the passé composé is used.
The imparfait may seem harder to know when to use, but it is not actually as tricky as it may seem. If you are wanting to describe something that you used to do, you need the imperfect. For example, in your directed writing, you may finish your essay by saying "I used to think that French food was terrible before my trip", to translate this, we now know it has to be "je pensais que la cuisine française était terrible avant de mon voyage", as your opinion is now different. Finally, if you are describing a repeated action, the imperfect is required as well. If you are talking about a job you had, such as "I worked as a waiter every day when I was there", it would be "je travaillais comme serveur quand j'étais là", as this action happened repeatedly.