For carne asada, you will need thinly sliced beef. Out of convenience, most people use flank steak or skirt steak to make carne asada here in the U.S. Out of the two- I recommend using skirt steak. For best results, I suggest heading to a local carniceria to purchase something called flap steak or ranchera de res. Apart from a carniceria, I have only been able to find non-marinated flap steak at Whole Foods, but it's usually quite expensive.