Starting out at 5th level, and I am not sure how to calculate attack bonus. The reason I have this question is because with the Martial Arts feat(?) it says that I would use DEX instead of STR for attack and damage rolls, so do I use DEX when calculating my attack bonus? Or do I still use STR for calculating my attack bonus (for an unarmed strike).

Strength is 11, so the STR modifier is 0 whereas Dexterity is 18, so the DEX modifier is 4. Proficiency bonus is 3.

So would my unarmed attack bonus be 3 or 7? Or am I missing something completely?