Make sure you use a concordance that matches the translation of the bible that you are using. Since the concordance is an index to a particular vocabulary, using a concordance for a different translation could be confusing since it references words that may or may not appear in your translation.
Most of you will be using the New Revised Standard Version (NRSV). Oxford Biblical Studies provides two online concordances, a concise NRSV concordance:
and a concise concordance to the NAB (New American Bible):