Killing cows is a great way to make money in Runescape, because they are fast and easy to kill, as well as dropping 500gp a kill (bones, raw beef, cowhide). Don't make the mistake of only picking up the cowhides, you'll make more money by picking up everything. You can make in the 80K -90K range with no skill levels if you are fast.
To kill cows fast it is advisable to use the best weapons you have. There is no level requirement to kill a cow, but to kill cows fast having at least 20-30 strength or range will keep the kills fast. Cows have only a small amount of hit points, so you should be able to 1 to 2 hit them.
The best place to kill cows is at Lumbridge. They are a 30 second run away from the castle. You can bank at the Castle, on the third floor. If you can use the Lumbridge teleport spell you can make the runs slightly faster, as well as gaining some magic experience.
The cow fields may be crowded at times so it is advised to world hop until you find a good world. Sometimes people only pick up the bones or only pick up the cowhides. Don't be afraid to pick up the other two items. It will speed up your trips even more.
If you have a low defense and are damaged while fighting cows, make sure you buy some trout beforehand at the grand exchange and heal yourself when necessary. Rangers can hide behind obstacles to avoid damage. Wearing metal armor is a bad idea because of the weight.
After killing Cows and collecting to your heart's content, sell your gains in the Grand Exchange to get your money.
Happy Money Making