SSDI (Social Security Disability Insurance) is a program that provides benefits to individuals who are disabled and have a sufficient work history, having paid into the Social Security system through payroll taxes. SSI (Supplemental Security Income), on the other hand, is a program that provides financial assistance to elderly, blind, or disabled individuals regardless of their work history, based on financial need.
If you want to hire an attorney you would enter a fee agreement with that attorney. This fee agreement would be on a contingency fee basis, which means they will represent you but only get paid if they are successful.
Yes, you have the right to appear at the to the hearing yourself and if you have a witness that has important information regarding your disabilities and your limitations you have a right to bring witnesses.
You have the right to appeal a judges denial if you claim to the appeals council and if the appeals council denies your claim you can then file an appeal to federal court.