The turnip is cooked like the carrot: steamed, glazed in butter, or in a pot-au-feu. Small new turnips are mild and are eaten almost raw; the big winter ones need longer cooking.
Firm, smooth, rather small, heavy for its size. A big spongy turnip is old.
Cut both ends. Peel with a peeler, small new turnips just get scrubbed.
One minute.
In the fridge, two weeks, without the tops.
The page "How to boil and steam vegetables": turnip wedges, fifteen minutes.