There are a lot of resources some have already been mention so I am just going to tell you what I use ( this is for french but a lot of then can be use for different languages)
Listening: NETFLIX or YouTube ( you should add chrome extension like Language Learning with Netflix they also have a YouTube extension.
Speaking: Itaki or HelloTalk (free apps) they are exchange language programs. Kinda like a penpal.
Reading: I do news articles. I would suggest to research online as I am sure there are resources specifically for your target language.
Writing: have a journal and use Itaki or HelloTalk
Other suggestions: use the app Anki. Anki is a free and open-source flashcard program that utilizes spaced repetition, which is a method used to increase the rate of memorization.
For free courses I think I saw coursera have a beginner level for mandarin but if you dont like it I am sure that there are a lot of other lessons in YouTube.
AT THE END OF THE DAY: the best recommendation is just to start. Dont over complicated :).