I make shredded beef tacos fairly often, I start with a bottom round roast trimmed of fat, salt pepper and garlic it. Sear it in a hot skillet and cook on low in a crockpot. after several hours I add 1/2 of a bottle of cheap lager or other low hop beer and cook for another hour or two, I add a couple cups of homemade salsa to the crockpot for the last hour of cooking and shred it back into the liquid in the bottom of the crockpot to keep warm. I usually simmer it in the crock pot for 30 minutes to get all the flavor in the shredded meat.
Fry up some homemade taco shells, fill with shredded beef and top with your choice of cheese, sour cream, lettuce, whatever you like really.