You are right, "nous nous mimes à causer" means "we began to talk". "Nous nous mimes" comes from the simple past form of "Se mettre". "Se mettre" means "to begin smth". Simple past is mainly used to express an action that is now finished and oftenly a brief action.

"Causer" is a familiar form of "parler de/ parler à", to talk about sth or to chat to sm. In this context it has nothing to do with the verb "to cause sth".

I don't know a lot of websites about french language but in France Larousse is a reference http://www.larousse.fr/dictionnaires/bilingues

You are right, “nous nous mîmes à causer” means “we began to talk”. “Nous nous mimes” comes from the simple past form of “Se mettre”. “Se mettre à quelque chose” means “to begin something”. Simple past is mainly used to express an action that is now finished and often a brief action.

“Causer” is a familiar form of “parler de/ parler à”, to talk about something or to chat to someone. In this context it has nothing to do with the verb “to cause something”.

I don't know a lot of websites about french language but in France Larousse is a reference http://www.larousse.fr/dictionnaires/bilingues