People often ask me for advice about how to learn English. As for it, everyone is different. I'll share some of my experience. I once took a school year English course in Wuhan University, but I mostly learned English on my own.
One thing I can tell you is that, once you've started learning, you should try to think in English as much as you can. If you see something when you're riding your bike or walking down the street, just think about "How would I say them in English?" At first, it might be with simple words or phrases, but later you should think about longer phrases and sentences. Now I'm in America, and for most of the time, I would think in English and speak it directly. It's amazing how much this helps.
For several years after I started learning, I was often afraid of talking with others in English because l didn't want to make mistakes. Finally I got it over. And while traveling in America, I'd try to hang out with friends who couldn't speak Chinese, so that I'd have no choice but to speak English. And I also decided that if I said something stupid and other people laughed at me, then so be it. After that, my English started improving much more quickly.
Besides, it may be a good idea to ask your American friends to look at the way you're writing, and give you some suggestions for improvement.