We are a Brazilian family in Brazil and therefore we speak Portuguese. I can speak English reasonably well but my wife is just learning it now. We want our 9 months old son to learn English, too. However, we only use Portuguese at home and there is no environment where we speak English naturally, so I bet we will have to resort to English classes (and help at home, of course).
There are a lot of questions about multilingual children here but none of them seems to approach the precise point I want to understand. So, my question is: what would be a good age to start to teach a child a foreign language? What would be good ways to teach such a language to a child?