Duolingo is great as far as a free learning option. There are ads that you can pay to remove.

The Knockout is a great detective/organized crime story. It’s on the longer side, but the pacing is consistent throughout and the time spent developing characters is justified by high quality writing.

I found https://www.hackingchinese.com/ to be quite useful.

I have also found learning about the radicals and general construction of characters to be useful as well, being able to look at them as a collection of things rather than just a jumble of lines. A good student’s dictionary is helpful here. It’s a big chunk of change but once you have it you have it.

Duolingo is OK, but you need to be disciplined with it, and it doesn’t help much with pronunciation or production.