Listen to, practice, and understand American pronunciation.
With jisho.org, you can enter romaji, hiragana, English, or Japanese. It's great, and it's free.
The Macmillan Dictionary uses British pronunciation, but has very clear definitions. So try it. You might like it!
With dictionary.com you can listen to pronunciation. It's free right now.
With Merriam-Webster, one of our oldest dictionaries, you can sometimes find English Learners' definitions. It has a lot of good stuff, and right now, it's free.
With Longman Dictionary online, you can get ESOL definitions, and listen to pronunciation both in British and American English. Free!