Im going to build a wall for a fireplace. What is the right ratio to mix my mortar at? How much is a part?

A part is a ratio expression, if it says 1 part water to three parts sand then think of it like a measuring cup, 4 parts total so 1/4 water and 3/4 sand. The morter mix bag should tell you the the correct mix for that brand. They are not all alike. You know that you need fire brick to line the inside of the fireplace, you cannot use plain brick as it will crack and leak smoke into the wall.