How do you know when the past participle needs an agreement and what are the different agreements?

All verbs which are conjugated with être will require the past participle to agree with the subject.  (That is to say the person doing the verb, eg. in the sentence, "Louis est allé au cinema.", Louis is the subject).

In the example above, the subject, Louis, is masculine singular and therefore no additional ending is required.  However, the sentence, "Emily est allé au cinema." would be gramatically incorrect.  This is because Emily is a feminine singular subject and so an extra "e" must be added.

The agreement endings are as follows:

Masculine singular: -              eg. Louis est allé au cinema.

Feminine singular: +e            eg. Emily est allée au cinema.

Masculine plural: +s               eg. Louis et Gilles sont allés au cinema.

Feminine plural: +es              eg. Emily et Maria sont allées au cinema.