I learned how to make chicken tacos from another Marine wife years ago and have made them many times since. A whole chicken works great but I have bags of chicken breasts in my freezer so that is what I used today.
Before I left for work this morning, I put a whole bag of frozen chicken breasts with a few stalks of celery, some dried chopped onions and enough water to half way cover the chicken into the crock pot. They cooked on high until I got home 5 hours later. I drained and shredded the chicken and put it in a pan with 1 chopped onion, 2 teaspoons cumin, 1 teaspoon garlic, a little red pepper, salt and pepper to taste, 8 ounce can of tomato sauce and 1 can of water. The cumin is really essential to giving it that Tex-Mex taste.
Stir well, cover the pan and let it simmer for 15 minutes or so. This will fill lots of tortillas or taco shells. It is great on taco salad and nachos too.
I like to make a big batch and freeze the extra in meal size portions.