Why is this word like this?
With present tense its
-ę
-sz
-e
-my
-cie
-ą

The verb żartować changes the root to żartuję.

I know ać is usually am but this changes the whole word. It makes no sense to the rules.

Question

Why is this word like this?
With present tense its
-ę
-sz
-e
-my
-cie
-ą

The verb żartować changes the root to żartuję.

I know ać is usually am but this changes the whole word. It makes no sense to the rules.

Answer

-ać verbs do normally conjugate to -am, -asz etc.

But -ować verbs (often) conjugate differently, to -uję, -ujesz etc.

Answer

Try this please: http://pl.wiktionary.org/wiki/Aneks:J%C4%99zyk_polski_-_koniugacja_IV 
In this article there is particular case of the "koniugacja". 
I cannot explain it better, I'm not any teacher. The table contains proper information. 
Greetings.