Where do I start with restaurant stock control if I have never done it?

Smallest useful step first. Open Menu, then Inventory, and count the two or three dishes that genuinely run out, such as the cheesecakes you made this morning. Live with that for a fortnight. Then add ingredients for the things that cost real money, give your ten best sellers a recipe, and you have food cost. Trying to do the whole store cupboard on day one is how people give up in week two.