I blind bake the pastry so keep it from going soggy, but it's up to you.
Basically a quiche is a savoury custard tart, but not a smooth filling, it's ok to just break up the eggs without beating them really well.
The eggs are what make it 'set'.
So, eggs are needed, then some sort of creamy liquid, milk is fine, cream is lovely and rich, a mix of both is fine.
Then, flavourings are needed, grated cheese almost a must for a tasty quiche, cheedar is nice, red leicester gives a lovely colour, goats cheese, blue cheeses are lush if you like that sort of thing, and mixed stronger flavoured cheeses work well too.
Veg can be added, sauteed onions, mushrooms, broccoli (blanched), tomatoes (raw) caugettes (sauteed), leeks, spinach (swiss chard too) capers...
Protein can be added (or the cheese can be enough): fish, smoked fish, anchovies, prawns, cooked bacon, ham, chicken chunks, cut up hot dogs or cocktain sausages (surprisingly nice), etc etc etc
It needs to be cooked on a low heat for a long time, ideally, until it is set and firm to the touch. The higher the liquid to eg ratio, the longer it will take to set. .
And, if you cannot be bothered with all that, spanish omelette can be made with almost the same ingredients minus the pastry