You should have learned about pronunciation since the beginning! It's important!
Anyway, it's not too late, don't worry.

If you think you're going to hear more American English than British English, or if you plan to go to the US, or if you just like American accents more, then I suggest you learn more about American English. That's what I decided to do too, and decided I would use an American accent, but after a while I realized... you won't just hear one accent though: if you pay attention to what you hear you'll realize there's actually a lot of different accents (from different part of the US, from the UK, foreign accents, etc.). That's why the more you learn about English in general, the better.
Starting with a general American accent is a good idea: that's what I did too. What I did was read "American Accent Training" by Ann Cook (and listen to the audio files that come with the course), and that boosted my listening skills. Before that, I hardly knew what spoken American English was, I'm afraid.
One warning: don't take any courses like that like "the bible" that will tell you how everyone is supposed to speak. I told you, the reality is that you'll hear a lot of different ways of speaking when you watch movies. However, that course is very complete so it's a perfect starting point, and it's probably more than enough for the vast majority of learners. Here's the official website. Good luck.
http://www.americanaccent.com/