Read books and the newspaper as often as possible, and watch the news and/or listen to NPR every morning (if you can). Head over to the closest public library. There's usually a Reference desk somewhere (or just an employee or volunteer who happens to be nearby) who can help you find a book you'll like.
Read, research on internet. Take courses (online/uni/other institution), watch documentaries. Some things can't be taught though, have to be learned through experiencing it. Getting lots of different jobs is a great way to learn things also.
just find what you're interested in and simply read; you don't always have to cram it into your memory if allowed enough time. i love anatomy, and in just a month I had memorized the entire Eye, nervous, and vascular systems (along with much more) just by looking at it over and over again, and not purposely trying to remember it