Bienvenido, Marito! We're glad you found Chef Talk.
If you make a marinade with some acid in it (citrus, tomato, wine, vinegar, etc.) the fibers of the meat will be more tender. There are other ways (pounding and chemical tenderizers), too. If you are using skirt steak, flank steak or other steak with strong fibers, be sure to let it rest for a few minutes after you grill it, then slice it thinly across the fibers for more tender slices.
I hope some professionals stop by to give their advice! I'd be happy to learn more myself.
Marito, please stop in the Welcome Forum and tell us more about yourself. Again, welcome to the Cafe!
Mezzaluna