I've had people ask me how I apply my eyeliner, especially since I have monolids. Below are some different examples of how you can apply your eyeliner, regardless if you have monolids or not.
(Sorry for the poor quality in my eyeliner application, I was kind of in a hurry and the pencil I was using isn't very good.)
This is usually my everyday look, just simple eyeliner on top and bottom. You can't really see the upper eyeliner unless I close my eyes though
Small cat eye winged slightly at the corners
Thicker, more dramatic cat eye
Double winged eyes
Lined all the way across on the bottom
Heavy, dramatic eyes
Obviously there are probably many more ways you can use eyeliner, but these are just some suggestions in case you don't know where to get started. Have fun and don't be afraid to experiement!