My kids did a little bit of everything. Movies, music, storybooks, tutors, apps, etc. DD is a natural language learner and knows 5 languages and continues to seek opportunities to learn more. DS was more reluctant, but eventually learned through tutors and immersion (travel).
For high school age, we used iTalki to find tutors in a Scandinavian language with marginal success. One of the kids found a high school student (native speaker) who was willing to work with him weekly because the tutor needed community service hours in his country to graduate high school. The tutoring sessions were great supplements, but ultimately, we found the kids needed more structure so we hired private tutor (via Skype) to take them through lessons from a textbook. We did that with the Scandinavian language with enough success to send kids abroad (independently).
For younger kids, look for a comprehensive curriculum specifically designed to help you, even if you don't already know the language. Calico Spanish is set up for anyone to teach Spanish to kids. They use fun characters and video based stories. Be sure to ask them for their homeschool discount.