Cow is a Combat Skill that is enabled after being cursed and transformed into cowform by attacking Maronesa. While in cow form you can level up Cow abilities, more specific information about the cow's abilities can be found here.
Reversing the transformationEditTo transform back into humanform you need to talk to Harry the Wolf who is located in the town in Serbule near the windmill and complete his Un-cow-ification quest. Alternatively Going back and beating Maronesa in the tutorial cave will also transform you back in to a human however that would require some Cow training first or the help of friends. Another way to beat the cow early on is to take advantage of the Endurance-skill spell Charm Rat and gather a small group of Giant Rats in the tutorial cave to fight alongside you.
This is what you will need to transform back into humanform. You can obtain grass by leveling up as a cow and unlocking the graze skill (level 5 cow) which allows you to get grass every 50 seconds or you could loot haystacks. To get bottles of milk you will have to buy water bottles from Theresa close to the windmill and then drink the water to get empty bottles. You will then have to find a non-cow player to use your empty bottles to milk the cows in town. Alternatively, you can level up cow to 23 to gain the ability to milk yourself in cowform, more about that can be read here. Mushrooms can be collected in the tutorial cave or out in the woods of serbule, they grow particularly well around the entrance to the tutorial cave.