A turkey sub is low fat, filling, delicious, and healthy!
There isn't really a hard-and-fast rule to making a delicious sub sandwich. If it sounds like it would be good, then it usually is.
So rather than give you a strict list of ingredients, I am just going to make some suggestions. Feel free to experiment and go all out with your ingredients. Use whatever you think sounds good. Keep in mind that turkey has a very neutral flavor, so there isn't much it doesn't go well with.
I personally enjoy my subs to be overflowing with veggies - if you're not making a mess, you're not doing it right!
Other Optional Toppings:
Take your French bread and slice it down the center (either all the way through, or leave a hinge.)
Add your turkey.
Top with cheese*, if you like.
Add any other veggie topping that sounds good to you.
Add the mayo, oil, vinegar and/or salt and pepper.
*If you decide to use cheese, you can leave it open faced and throw it in the broiler for a couple of minutes to make a delicious Turkey Melt!