Oil your grids and preheat your grill to 400°F.
Coat the chicken in the fajita spices, reserving 1 tbsp. for later. Slice your onions and peppers in half, skewer the garlic so it doesn't fall through the grids, brush them with oil and dust them with the remaining fajita mix.
Grill the chicken for about 7 minutes per side, and the onion and peppers until they are all done. The peppers and onion will be done before the chicken. Set them aside to cool a little before slicing.
When the chicken is cooked all the way through, slice or shred it.
Serve in a build-your-own fashion, allowing you and yours to put as much or as little of everything on their tacos. I recommend a little of everything, start with cheese, then hot meat, peppers, lettuce, salsa and sour cream.