Looking for a flatshare or furnished apartment to rent

The best way to find out what is actually on the market is at:

https://www.immobilienscout24.de/

This is the most popular German website for finding apartments.

I did a search on the site with the criteria Düsseöldorf up to 600 Euro and found quite a few listings. The prices are all listed without utilities (Kaltmiete) so if your budget is with utilities (Warmmiete) then you'll have to go with something that lists less than 500 Euro rent. But for this price I do see what appear to be OK small apartments of say 30 to 40 square meters that should include kitchen, bathroom and living/bedroom.

But like Beppi mentioned, it is highly unlikely that anyone will commit to rent to you before you arrive. One should make a good personal appearance, show documentation that you have a steady job and be prepared to put down a 3 month deposit.

Something to watch out for is to make sure it shows "Einbauküche" which means built in kitchen. Otherwise, apartments in Germany are often rented with a kitchen space with nothing in it, no cabinets, no sink, nothing! People who rent long term often like to build in one according to their own taste - or have something from the last place they lived. Coming from overseas, you are not going to want to have bring in your own kitchen sink.

Otherwise, it is usually best to find an unfurnished place and if you have a limited budget you can find used furniture or go to places like Ikea. Furnished places are not common and generally way overpriced. This means that it is often still cheaper to buy furniture and just leave it behind even if one only stays for say 6 to 12 months rather than pay triple the normal monthly rent because it has some furniture.

Which areas might be good will depend on where you work and the proximately to shopping and public transportation. I don't know Düsseldorf well enough myself to advise about that. Getting around German cities is usually best with public transportation. And sometimes it can be better to have an apartment that is say 8 kilometers from your place of work but convenient to a major subway line than to live 3 kilometers away with bad or multiple connections.

Another option for young people is to get a room in a shared flat. Best possibility for this is to look at the pin-boards at the University. Such a living situation can vary greatly; say  having one or two house mates that are a bit older rather than a bunch of really young party animals. Also, students are more likely to speak decent English and not mind dealing with a foreigner than the general population or many renters. The advantage is that it can be cheaper and one has social interaction and people that can give you advice about how things work locally. But better to live alone that be stuck with flat mates one doesn't get along with well.