How do i get the name of my company registered or trademarked?

I HAVE A COMPANY HERE IN TECATE,MX. AND THERE IS A COMPANY IN SAN DEIGO WITH A SIMILAR NAME . THEY ARE GIVING ME PROBLEMS, SAYING THAT I CAN'T USE MY NAME ANYMORE BECAUSE IT IS TO CLOSE TO THEIRS. SO HOW DO I GET MY COMPANY NAME REGISITERED OR TRADEMARKED TO KEEP OTHERS FROM USING IT?