I eat at Subway pretty frequently. I would like to choose the heathiest ingredients to put inside my sandwich. What are these ingredients? Assume that I am a normal person with no illness like diabetes.
The American Heart Association, as well as the NIH and others, have recommendations on eating healthier when eating out.
The fundamentals of what specific foods AND what quantities of those foods make for a "healthy" diet involves a very large discussion that is beyond the scope of this question.
In this specific setting, one way to approach it is to consider the choices between nutrient-dense foods versus calorie-dense foods as ONE measure of "healthy." For weight management, the number of calories you eat each day are important to consider as well.
Nutrient-dense foods have more nutrition packed into something with fewer calories - for example spinach, cucumbers, peppers, tomatoes, onions, sprouts... Olives and avocados have more calories but are still nutrient dense with healthy fats.
Calorie-dense foods include things that have many calories (and often fat) in small amounts of the food, proportionally high in comparison with less nutrition - like mayonnaise, fried foods, sugar, flour/bread, etc.
Examples: Many cheeses and some meats (like salami or tuna with mayo) have more calories and fat per gram of protein and are therefore less nutrient-dense than lean meats (like chicken or turkey breast). Mustard or vinegar have nearly no calories, whereas Mayonnaise has a lot.
Subway has a nutrition information webpage for specific information.