How do you say I am angry in Japanese?

Well.. 'Mendokusai' is irritating or bothersome.

I doubt you would want the polite form of you, 'anata', so.. You could try the slightly-rude depending on the situation and your tone 'Omae wa' or the ruder 'Temee wa'. If you really want to be rude, 'Kisama' can work as well.

Kisama these days roughly means 'wretch.'

So in order of rudeness:

"Anata wa mendokusai!"

"Omae wa mendokusai!"

"Temee wa mendokusai!" or

"Kisama wa mendokusai!"

Hope this helps.