Is there something u can take otc before a big presentation if u have extreme nervousness in presentations?something that makes heart not beat so fast

Not OTC, but you can: get a prescription for low-dose propranolol , 10 mg., 30 minutes-1 hour before public speaking, if your GP finds no medical condition that contra-indicates its use. Try it before the day of your talk to make sure it doesn't make you feel light-headed. See http://www.consumerreports.org/cro/2012/04/can-you-treat-stage-fright/index.htm.