As per my previous message, for what soil to use, everything you need should be answered in this very popular step-by-step article by resident Bunnings Workshop horticulturalist @Noelle - How to fill a raised garden bed
What to plant really depends a lot on what you want to eat! I'm assuming you want to grow herbs and/or vegetables? In Melbourne, it's not too late to plant favourites like asparagus, beetroot, broccoli, capsicum, pumpkin, potato, rhubarb and tomato, along with a range of herbs such as chives, dill, coriander, rosemary, oregano, basil and parsley. Alternatively, you could grow some colourful flowers?