When im nervous I start breathing very fast and my heart starts to beat faster? What can I take to block that?

Get, not take. Nothing negative meant; however I hope this helps you realize the best is to get a thorough psychiatric exam so that you may understand the cause as well as treatment choice. Most likely discussion of combined use of med. : anti-anxiety and/or antidepressant + psychotherapy. A full physical exam should be incl. To check possible contrib. Cause & effect.
Several things. First---if you were to try waiting and breathing, it should pass, that being said, I would not take anything over the counter without checking with you doctor. Benzodiazepines like Xanax (alprazolam) are ok if used for the short term. For recurrent anxiety like panic attacks, antidepressants like zoloft are used. For anxiety like speeches, we use Inderal a BP drug, talk to your doctor about these.