Creating soil for a raised bed is simple, nature does it every day, it just takes awhile if you do not give a helping hand. The general scheme of soil creation in nature is grow a plant, kill a plant, decompose a plant, and rejuvenate the soil.
We can all do the same thing, we can let someone else do the work and buy compost or topsoil in bags. This is the more expensive option but if time is limited and you want to garden right now it is the fastest option. Just buy your soil put it into your raised bed and start gardening.
Some towns compost yard waste that is left at the curb and if you come and load it yourself you can take it for free. Check with your local sanitation department and see if it is available to you.
For those people who have more time than money, creating compost is an ideal solution. Compost is simply gathering dead plant material piling it up and waiting. Stirring occasionally just like soup. You need a mixture of regular plant material and woody plant material to get your compost pile heating up. Wait a couple months with weekly tossing and poof, compost for your raised beds.
If you do not purchase any bagged garden soil you will need to mix the compost with some native soil, which will need to be replaced yearly. The compost not the native soil, compost bits tend to shrink until they are practically gone but the soil they leave is better than the soil that you started with.
If you do not have time for stirring, and no money for the buying of bagged dirt, another option is called lasagna gardening. That is simply the lazy man's compost. Instead of building bins and turning you simply pile the plant parts onto cardboard or sheets of newspaper in the area you are planning to garden in and wait.
The newspaper and cardboard will keep the weeds down and the plant material slowly decomposes to enrich the soil below. You may still need to add some native soil from another area of your property if your lasagna garden doesn't fill your raised garden frame. This method will take about a year to decompose completely.
One of my new favorite soil creation techniques is piling sod that I have removed for the creation of garden beds and just letting it sit and the grass and roots decompose while the worms mix it all together. I just pile it anywhere but it tends to become a weedy mess if you do not keep on the weeds. Probably drives my neighbors nuts but I don't notice them any more than I notice the weedy mess at the side of my home.
Tunnel vision can be a nice thing. What dirty dishes, I don't see any. J
Any of these systems can be mixed and matched to suit your time, budget, and lifestyle needs. Creating soil for a raised bed is simple, give it a shot.