someone tells me how to play donkey derby?

[Nerdazoid]

Sometimes I get points for pinning tail on donkey without playing. Does that mean I am a donkey?

No, you are actually winning points from the "Pin the tail on the Donkey" promotion and not the Donkey Derby. When someone else places a bid on being the donkey either they place in the top 3 and get a piece of the prize pool, or they lose in which the money they paid to be the donkey gets distributed among everyone who lasted longer than him in that game.

So you don't have to do anything special to get those points except last longer than the donkey.

Does that mean I am a donkey?

Whether you're a donkey has yet to be decided. But as far as the promotion, no that doesn't make you the donkey.