It isn't a word. Understand it when you read or hear it. Don't try to use it. You see it in comic strip talk balloons. You might see it in quoted dialog. You might see it when someone is writing but using colloquial English (online forums, text messages). It is an attempt to convey the sound someone makes when they are disgusted. It is the sound someone makes when they are trying to imitate the sound of vomiting. Because it isn't a word, just a phonetic transcription of a noise, there isn't any right way to spell it. It might be spelled "Aagh!" or "Argh!" or "Arrgggh!" or "Aaaaargh!" It might be followed by more than one exclamation point.

It's usually used to express someone's frustration about something when texting. Eg " Aargh! My brother is so annoying"