How do you get the cutie mark in my liittle pony adventures in ponyville

You must max out all of the Friendship stats:

-Maxing them all once will earn you a Cutie Mark and unlock the Pegasus pony type.
-Doing it twice will earn another (I believe) and will unlock the Unicorn pony type.

The most difficult to increase will be Kindness; Applejack's minigame is the only way to do this reliably. I recommend doing it on the Easy setting, collect apples until Snips and Snails have been chased away about 5 times, and then let the game end. Repeat until Kindness is full. It takes forever, but it's the quickest way.