I am an american recently admitted to the Universitat Autónoma Barcelona for a master's degree in direction of public relations an comunications agencys. I want to know if I can get a scholarship and what do I have to do to obtain it?

Didn't you research this prior to choosing this university for your Master's? Ask the financial aid office at the University for help. They may have private donors willing to help students with your major.

Many places do not have "free money" only the school office would know what is available.