Do you want to learn American usage or British usage? If you want to learn American English, I would recommend the *Longman Learner's* dictionary or the *American Heritage Dictionary for Learners of English* [be sure to include "for learners of english" when you search for the second one. It is different from the standard American Heritage Dictionary, which is written for natives]. The American Heritage dictionary has wonderful example sentences, and the Longman dictionary has very useful supplementary material. If you want to learn British English, I would recommend the *Oxford Elementary Learner's Dictionary* and the *Oxford Advanced Learner's Dictionary*