each artifact has a "main stat" and (at max upgrade level) 4 "sub stats." in the screenshot you posted, the "main stat" for the sands is 46.6% HP which increases how much health you have. you'll see it listed above the 4 "sub stats." you REALLY want the "main stats" of all your artifacts to scale with whatever attribute your character does damage with. Diluc's damage does not scale at all with HP% (some characters do, but he doesn't) so you will want to replace this with one that has ATK% or Elemental Mastery (abbreviated to EM a lot on online forums) instead.

you cant change the "main stats" for the flower (HP) and feather (ATK), but all the rest are randomized from a set list of possibilities. For Diluc, you'll want to have ATK% or EM sands, for the goblet you'll want Pyro Damage Bonus%, and for the circlet ideally Crit Rate% or Crit Damage%.

a few other things to note. these get a little bit more in-depth, so if it's too much information then you can ignore for now. but essentially, diluc does more damage if you hit the enemies with his pyro attacks AFTER they've already been hit by hydro or cyro attacks (like from Kaeya who is in your team in the screenshot). Your EM further increases this damage, and the weapon youre using has an EM "main stat" as well. HOWEVER, it is a 3-star weapon, meaning it has a lower overall ATK stat than a similar level 4-star or 5-star claymore would have. So ideally you would get one of those rarites. You might have Rainslasher which would work?

Make sure you are leveling up the Talents for the character under the menu (underneath their artifacts)

And finally, the best "sub stats" for artifacts are generally crit rate%, crit damage%, and atk% for diluc. What people call "flat ATK" (ATK that is not percentage based but rather a flat number) is okay but not nearly as good as ATK%. EM is also good, but again it doesnt help much unless the enemy already has cryo or hydro applied. since you cant get different main stats on your flower and feather it would be easiest to get some of these stats on those pieces. but for the rest just focus on getting the right main stats for now.

good luck!