i am going to mediation tomorrow im suing my past employer for unpaid overtime wages, what should i prepare

You should be prepared to show proof that you worked the hours in question and have your pay checks ready to demonstrate that you did not receive your full pay for all the hours that you worked.  At trial, the burden will probably be on the employer to prove that you didn't work the hours that you claim if you testify that you did work the hours.  The Department of Labor Regulations place the burden on the employer to keep records demonstrating the time that you worked, so the employer may have a difficult time disproving your testimony.