It depends on what you are trying to do. You will need one if you want to transfer assets that are titled (such as real property) into the trust; open a bank account in the name of the trustee; transfer any investment accounts in the name of the trust, etc. The Declaration of Trust is the document (aka Trust Agreement) that creates the trust and defines its terms and who gets what. If you already have the trust, a trust declaration or agreement has already been drafted. Additionally, if you want to have any changes made to the trust agreement, you will need to have an attorney draft an Amendment or Restatement of the trust and have it executed with Florida's required formalities.
The declaration of trust is your trust. The certificate of trust is not needed but can help keep things private and provide a easier way to open bank or stock accounts.