there is plenty of possibilities to learn German online. If you want to get an overview of free tools than I can recommend you this website:
It contains a list of web-based programmes that you can choose from depending on your current level and what you want to focus on while learning (for example grammar or vocabulary etc.).
There are three big sub-categories:
1. "Gezielt lernen" -> there you can choose one of the other sub-categories to work on your German in a very focused way.
2. "Nebenbei lernen" -> this list contains programmes and websites that you can use to learn in a less strict/systematic way. Some of the offers are more fun-based.
3. "Effizient lernen" -> here you can find some other useful tools to improve your German.
All of the linked programmes were rated by the community. So you can easily see which of them is helpful or not.
Let me know if my answer was helpful to you or if you have further questions.