Viselia (https://viselia.io/) helps travellers prepare that file. Its guide to an Australia visa from Qatar (https://viselia.io/visa-des ...) sets out what you generally need: a passport valid for at least six months, a clear purpose and travel itinerary, evidence that you can support yourself and that you have ties to Qatar, and a letter of invitation if you will be staying with someone in Australia. Viselia can help prepare the ImmiAccount application, a cover letter and the itinerary. You review the file before anything is finalised, and you submit original documents yourself.
No agent can guarantee that a visa will be granted. That decision belongs to Home Affairs. The government application charge is paid to Home Affairs, not to Viselia, and should be confirmed on the official pricing estimator, because fees are updated from time to time. Processing times vary by stream and should be checked on the Home Affairs processing-time page before you apply. The subclass 600 visa does not allow work.
Viselia also publishes a free checklist and document templates for this route, which you can use without creating an account.