I accidentally poked my eye with my fingernail. What should I do?

See an eye doctor. Right away, or go to the local er. You could have a corneal abrasion which untreated could lead to a serious infection.
Probably Nothing. If your eye doesn't hurt, isn't red and the vision clear don't worry about it. If you're experiencing discomfort, you may have an abrasion. If it persists, be sure to be seen by an ophthalmologist.