The main ingredients for the mince vary considerably, but at bare minimum you need
1. Some kind of protien, pork is the most common, shrimp is common, leftovers are entirely acceptable, marinaded tofu works.
2. Filler, usually a mix of shredded carrot, blanched rice noodles (a handful at most), diced mushroom (especially with pork). The key here is not to introduce liquid, the rice noodle will absorb some of don't boil the shit out of it.
3. Seasoning
Usually garlic, fish sauce (a tablespoon), i use chives, white pepper is pretty standard, salt. Herbs are better added to the sauce or a salad in most cases.