Staying Sydney or Melbourne?

Hi

I'm thinking of staying in Sydney/Melbourne for a while. Haven't decided for how long maybe 3 months to 6 months or even longer if the city is suitable for me. But during course I may need to have a part-time job to keep myself up everyday. Where or which website would you suggest for locating such job? Do I need to apply for a working visa first? And how much will be the living costs here? Hope to get some idea here. Appreciate for any suggestions. Thanks!