In French, if you want to talk about an action which occured and which has completed; for example 'Jake ate the apple', you would use the passé composé. This is a specific event which has taken place. You would use the passé composé when telling a story or when listing events which had happened, for example: 'I got out of the car, closed the door, walked towards the house and went inside.' These are all specific events which have occured and which would take the passé composé.
The imperfect however is used mainly to provide background information, for instance, what the weather was like, or what other people were doing at the time. The imperfect is also used to describe habits; in English we might say 'I used to swim'. You would also use the imperfect to describe a scene e.g. 'Le ciel était gris, et il pleuvait'.
There are other situations in which the imperfect is frequently used, like in if clauses with the conditional.
To form the passé composé you require an auxiliary verb which is either avoir or être, and a past participle. For this tense the auxiliary very is always conjugated in the present tense. To form a past participle for a regular -ER verb, you remove the -ER from the infinitve form of the verb and add -é. For example the past participle of 'sauter' is 'sauté'. For -IR verbs, you simply remove the R, and for -RE verbs you would remove the -RE and add -U, for example 'rendre' becomes 'rendu'.
For the majority of the time, you would use avoir as the auxiliary verb but here are also many occasions when you would use être. You use être as the auxiliary verb when you are using an intransitive verb, which is a verb which doesn't have a direct object. For example: 'il est sorti de la maison'.
You also use être with all reflexive verbs, for example 'je me suis lavé'.
To form the imperfect:
Take the present tense nous form of the verb, remove the -ONS ending and add the followin endings;
je - ais
tu - ais
il - ait
nous - ions
vous - iez
ils/elles - aient.