Starting a saltwater tank?

Hello all! My husband and I recently just inherited this freshwater fish tank from his grandmother. It’s very large and it was absolutely filthy when it was filled with water, and quite frankly still is so we’re still in the process of cleaning everything out. Of course keeping it freshwater would be easier but I honestly like the idea of converting it to saltwater more.

Neither my husband nor I have ever owned an aquarium or even lived with one in the house for that matter, so we don’t know the first thing about fish tanks. Does anyone have resources that would be able to help? I’m completely lost here :( the family that owned this tank doesn’t know much either and the little that they seem to know I don’t trust since they took horrible care of it in the first place