What should I do after finishing the tree?
Dzień dobry. I've conquered the Polish tree. It wasn't hard in case Russian and Lithuanian (I was surprised when Lithuanian helped me at some points) are my native languages so I have a huge benefit in understanding how Polish works. Still, I'm at the beginning and I have no idea what should I do now. Any tips?
After finishing the course, keep the tree gold for a bit, to let Duolingo's spaced repetition algorithms help you revise the stuff you've learned. At the same time, start reading Polish children's books, and listening to children's radio programs. If you can get a book and audiobook of the same thing, that can really help to improve both your reading and your listening skills.
A lot of people on these forums recommend using Memrise courses to expand your vocabulary, and Lang-8 for improving your writing skills.
You'll probably also want to find someone to speak Polish with. Have a look at this post about how to find language exchange partners: https://www.duolingo.com/comment/1444346. There are links to some useful websites on there. Don't miss DamedesLangues's comment, which has links to more sites.
It's also worth looking on the Multilingual learning resources page on the wiki.