Watch the films in the order they came out, beginning with Iron Man, and add the series once you are hooked.
There is no trick to this and no correct answer that unlocks the rest. There are two reasonable orders, and one of them is better for a first time.
Watch things in the order they came out
Start with Iron Man from 2008 and move forward. This is the order the films were made in, which means it is the order they were built to be understood in. Nothing is explained before you need it, references land when you have already met the thing being referenced, and the surprises are still surprises.
The alternative, watching in the order the story happens, sounds tidier and is worse the first time through. It opens with a film set in the 1940s and another set in the 1990s, both of which were made years later and are quietly full of callbacks to things you have not seen. Save that order for a rewatch, where it becomes genuinely interesting.
Decide how much you want
You do not have to watch everything, and most people should not start by trying.
Films only. The main run of feature films and nothing else. It is a complete story on its own, and it is the smallest thing that still makes sense.
Plus series. Adds the streaming series that carry the main plot forward. From 2021 onwards some of these are not optional if you want the films to make sense, which is a real change from how this used to work.
Plus everything else. Adds the shorts, the specials, the older television, and the films made by other studios about the X-Men, Deadpool and Spider-Man. Worth it once you care.
Pick the middle one if you are unsure. You can always go deeper later, and going deeper later is more enjoyable than giving up early.
What to do when you are lost
You will hit a film that seems to assume you know something. That is normal, and it is usually not your fault: the material was made over eighteen years by people who changed their minds.
When it happens, the useful move is not to stop and research everything. It is to keep going. Almost every gap is filled in later, on purpose.
Show detailsIf you want the honest version
Not every gap is filled in later. A handful of plot points were set up and then abandoned when plans changed, actors left, or a studio deal expired. Two decades of this material contain real loose ends, and no viewing order fixes them.
Where that happens, the entry for that film says so.