June or July is fine. Earlier is better but make sure you're prepared before you take it.
Materials - Transcripts for all places you have attended after high school (this includes medical school I think?). Activities such as volunteering, shadowing, clinical, and research, personal statement, money/credit card for applications, letters of rec preferably with DO letter.
It's not a whole lot different from AMCAS, the personal statement prompt is slightly shorter and class designations are different.