If you are a teenager and looking to improve your Mandarin, here's how you might do it.

Check with Your High School

If your school offers a Chinese course for you, then take it! Most school boards take their language education seriously, and it will be great to have classroom instruction so you can have plenty of opportunity to use the language in class. Most Chinese classes in high schools also teach through interactive activities and projects, and make use of new technology. And best of all, it's free.

Free Online Video Courses

Here are some, self-paced video courses you can take:

1. Chinese for Beginners by Coursera. Cost: Free

This is a course created by Peking University and taught by Xiaoyu Liu. The course is very structured, although my first impression is that it's a bit dry. But it's a great start, see if the teaching style matches with your learning style.

2. Yoyo Chinese. Cost: $16/month subscription.

Yangyang Cheng is a great Chinese instructor famous for her YouTube channel. Her learning product Yoyo Chinese offers free videos and premium videos for subscribed users.

Seek out opportunities for conversation

You must practice the language to get good at it. Seek out Chinese friends for conversation in your community. If you would like to seek out friends online for language exchange, give italki.com a try.

Get Feedback

Sometimes learners make mistakes, and without someone correcting them, it's hard to make improvements. Lang-8 is a service that give you the opportunity to write sentences and have native speakers correct you. Link: http://lang-8.com/

Take Standardized Tests

How do you know you actually learned the language to a certain level? Taking standardized tests is a great way to gauge your progress and help you target weak areas in your learning.
Take the Youth Chinese Test (YCT) is a test specifically designed for teenagers up the age 15. This is the official test sponsored by the Chinese government - very useful to get certified. It's evquivalent for adult learners is the HSK (Chinese proficiency test).
Official test website: http://www.chinesetest.cn/

Video courses for Intermediate Chinese

Once you move beyond the beginning stage, you may find it hard to find any more advanced Chinese courses.

Chinese Zero to Hero! offers intermediate Chinese courses that are complete and comprehensive. Give it a try at http://www.chinesezerotohero.com/