What spices are used to make a mexican fajita seasoning?
Penzeys (a spice co.) makes theirs from:

salt, black pepper, paprika, Turkish oregano, cayenne pepper, garlic, celery, Mexican oregano, basil, nutmeg, cumin, marjoram, thyme and rosemary. 

No numbers are given, and you probably don't need two kinds of oregano, but I've made it before with a similar list, and it is generally insensitive to precise ratios. I'd start with

2x black pepper, paprika, oregano, cayenne, garlic, 

1x basil, cumin, marjoram

1/2x nutmeg, thyme, rosemary

bulk up with salt (store-bought stuff is mostly salt), and experiment from there. You can probably find more on google. Add hotter things or anything you find interesting :)