Follow the procedures for reconsideration which should be in the documents you received with the denial of SSI.  If you are unsuccessful with reconsideration and have exhausted the appellate procedure, then the case can be heard by an administrative law judge.  You should present all of your relevant medical records to show that you are unable to work and any additional supportive evidence at the hearing before the administrative law judge.  Don't miss filing deadlines for filing your documents with the court prior to the hearing or your claim may be denied.