Yes you can. If you're intelligent enough, I think you can learn almost everything you want without a teacher, a teacher would help only if he/she is a native speaker. In my country they teach it at school, but the teachers are all stupid, I think even I know more about English than them, I'm not saying there aren't any good ones here, but I never found any
Internet helps a lot, there are tons of helpful information on the Wikipedia, Wikitionary, British Library and at BBC's site. The IPA symbols are a must if you want to learn the English pronunciation; without it, I'm lost.
As for me, my English is incredible poor, I've got all the information I need, but I'm too lazy to practise, especially the pronounce; since I don't have anyone here to speak English to, I don't usually do it because it's embarrassing to keep talking or reading texts in loud voice by yourself when there are people around you ^_^