There's no set rules about how or what to fill your raised beds with but we'll try and give you a few ideas and suggestions.
The first thing we recommend is to put down a layer of weed control fabric
. The indiginous weeds already in the ground where you intend to site your raised bed will welcome the fertiliser that you use to grow your vegetables and in no time will have put it to good use in making themselves strong and healthy. By putting down a weed suppressant fabric you'll keep them at bay for a good few years.
You might want to consider the drainage of your raised bed. Is the existing ground heavy and water-logged? Is it sandy and free-draining? A way to aid drainage of your raised bed is to put a layer of aggregate (large gravel) under your raised bed, a couple of inches deep. Remember though, your raised bed will usually drain well because of its elevated position.
To ensure your raised beds are fertile it's a good idea to mix some well-rotted manure in with your compost or topsoil when you fill your beds. You can fill the beds right to the top as the compost will settle over time. Finally, check the pH level of your soil and add a sprinkling of lime if it's too acidic.