If you want to learn English pronunciation online, than talk to Americans through some voice IM. It is not the best solution, but can be done …
using it in daily life.read book without dictionary.you must learn to understand what you read.if u give up,u can see dictionary
Along with using an online learning tool, you should watch American Movies, especially Animated movies, where the dialogue is simple and clear. You will learn it fast!
The best way to learn any foreign language is to speak with foreigners as long as you can. And when you will think using foreign language it would be a good result in studying it. But to my opinion to learn the English language is better on its motherland than in the US.
If you wanna learn and speak english then the best way is to do it by yourself. I would also advice you to watch some English movies and TV serials so you get acquainted with the accent everyday. If possible make sure you have subtitles while watching the movies in your local language (if your main language is not english), which will also help you a lot. You should also read english books to make your english better
You don't need to go or stay in America to learn American English. Take your time to study the dictionary and use some tools online.
watch english movie and have a small dictionary with you. reading an english book will help too. And don't shy to ask someone who is good in english (this method help me alot)
Watch American TV programs in your cable TV (HBO, Cinemax,etc.) You will learn a lot on how particular words are pronounced.
First you'll have to make yourself familiar with US English. For that you can watch some videos or movies as evaadams said. It will make you familiar with the way to speak and after that you need to enhance your vocabulary.