I know somebody who does fantastically well at pub quizzes - sport, history, the arts, current affairs, he knows the lot.
But he can't have an in-depth discussion on most of those topics, he just has a great memory and has learnt the answers to typical quizzes by rote.
There are a lot of blaggers out there. Anyway, get a quality newspaper and read it from cover to cover every day. You may not be able to follow news stories straight away, but over the weeks you will get a good grasp of current affairs and politics.
Read a tabloid to keep you up to date on sport and current gossip about the soaps and celebs.
Watch documentaries on television. If something grabs your attention, get books from the library and read up on the subject.
Wikipedia is excellent because it gives links to further reading on a subject, so although the Wiki page itself may be inaccurate the links are usually good.
Don't forget the radio - there are some first class programmes that discuss a wide range of subjects.