In my opinion you have to read! Read a lot of non-fiction and the most subjects the better! Usually the gateway is history, because you can picture things, you can what a historical movie, play a historical video-game and in a sense "relieve history". Start with topics that really interest you and expand like a knowledge tree. For example, if you are interested in philosophy, it is extremely important to fully comprehend it to read about history, economics, biology, religion, politics... Those are all connected and the biggest your amplitude of knowledge, the deeper as a person and intellectual you are.
If you focus on only two or three disciplines you are going to be the stereotypical American that only knows about two or three different topics a great deal and is totally ignorant and not curious about anything else.
I have been living in America for over two years and a half and yes, it is much, much better than where I come from, but this lack of curiosity, extreme pragmatism and specialization is by far the most annoying characteristic of the country, as you usually will have only one shared interest with the person and therefore can only talk about one topic, because if you switch to another one they know nothing about it and will either stop the conversation, come back to the original topic or just look at you saying "yes", "interesting".
As an example, I am kind of a gun nut and you have no idea how frustrating it is to see people with no amplitude of knowledge in their own field: I go to a gun shop and pick up a M1 Garand, a Mauser 1898 or Sturmgewehr 44 and the guy behind the counter knows everything about the mechanisms and so on and so forth. If I start talking to him about how those changed the nature of war, military tactics, engagement philosophy and whatever, they look at me as if I am a ghost and remain silent or say that they know nothing about it. Because of this damn specialization thing of focusing on two or three topics and caring about nothing else (for example guns, video-games and football). Or the continuity of art! Someone is a movie fan and after a while you switch the topic to music and they know nothing about it or vice-versa. Argh!!!