English can be a particularly difficult language for this, I think. There are so many exceptions to the rules that it can be a bit of a nightmare. They use phonics to teach children the rules and then spend years explaining all of the exceptions.
Lots of listening practise will help a lot. People are also usually quite nice about mispronunciation. If you're ever not sure just ask someone to correct you. Personally, I don't go out of my way to correct the pronunciation of non-native speakers, as long as I understand what they mean. However, if they ask me to tell them I will let them know.