what do you use to hit your dexterity score or spell attack? If it is your dexterity score do you get your + to damage bonus ?

It's a simple melee weapon you're proficient with and it has the finesse property. This means you can use your Strength or Dexterity modifiers for your to hit and damage rolls and you can add your proficiency when rolling to attack. It's basically a dagger that does 2-5d8 psychic instead of 1d4 piercing.

Shadow Blade :
It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60).

cool ty