Most people buy the little packets of taco seasoning from the store. They're cheap and easy but it's just as easy to make a batch of seasoning and it is much easier to control the amount of salt. I like to make a batch of this and keep it in the cupboard.
1/4 cup dried oregano
3 tablespoons dried chile powder
2 teaspoons cumin
2 teaspoons garlic powder
1 tablespoon dried onion
1 teaspoon salt
Mix together ingredients thoroughly and store in an airtight container.