If you or your partner is a native speaker of a second language, you should begin speaking to your child in this language as soon as possible. According to "Ask a Linguist," children start acquiring language rapidly around the age of 10 months, and can assimilate multiple languages well during this time. If neither you nor your partner speak a foreign language, a friend or relative who speaks a foreign language can introduce your child to the language at a young age.
Though exposure to a native speaker is an important component of learning a foreign language, children should learn the language from more than just one person. Therefore, speaking with and being exposed to relatives or teachers who also speak this language will reinforce the child's language skills. For this reason, immersion language schools that start children very young, even as young as 3 years old, can present a good option.
To make sure that the child learns more than just how to speak a foreign language, exposing him or her to other sources, especially those with a strong visual component, is an important aspect of language acquisition. Reading picture books to the child in a foreign language, watching videos featuring native speakers, and engaging in other visual exercises can reinforce the spoken aspects of the language acquired elsewhere.
As your child gets older, using the foreign language in context will prove important to becoming truly fluent or bilingual. Spending an extended period abroad, whether on vacation or even for a semester of a school year, will help your child put the language skills he has learned into context as soon as possible.