In my team for the story, i would put a Tyranitar, NOT mega, probably with armor. Can you give me an advice on nature?

For an in-game team, Adamant. Armor!Tyranitar is so bulky and hits so hard with physical attacks, but isn’t really fast enough to warrant much speed, and doesn’t have recovery to take advantage of a defensive nature. Alternatively, if you’re planning on giving it a special move or two (Ice Beam or Fire Blast), use a speed-lowering nature like Brave or Quiet to preserve its Special Attack.

What do you sugget me?

You have two main options, both with Armor.

TANK: Adamant, full Attack and HP investment, and then Stone Edge, Crunch, Earthquake, and one of Ice Punch, Superpower, or Custom Move Fairy/Flying.

SWEEPER: Jolly, full Attack and Speed, Stone Edge, Crunch, Earthquake, Dragon Dance.

Personally, I’d do the tank, since there are better (namely, faster or priority-using) sweepers that you could use instead if you want that. Tank Tyranitar is a defensive and offensive monster. For the ability, I’d recommend Sand Stream for that nifty Special Defense boost and chip damage unless the rest of your team really hates Sandstorm.

My Story-Team is: Eevee (obviously mega); Electivire; Delta Scizor; Flygon (probably mega); Delta Haxorus; Tyranitar. What do you suggest?

As I said before, Armor Tyranitar.

What nature?

Adamant. Go for a tank. Check my previous reply for details.

Only a thing, i think Tyranitar shouldn’t get the boost on spdef in the sandstorm, because the game is upated to 6 gen, i think

The Special Defense boost was added as a mechanic in Gen 4 and still exists through Gen 7.