Thanks for contacting Ask Extension with your question about raised beds. A raised bed soil needs to be a light, well-drained growing medium. The ideal mixture typically includes a combination of compost and garden soil. You should aim for a mix that comprises 25% to 50% organic matter, such as compost, by volume which enhances nutrient retention and improves soil structure.
Soil can be purchased or you can prepare your own mix. 
Pre mixed soils: You can purchase a soil mix locally. Good landscaping companies offer separate mixes for different uses, e.g., one with coarser soil may work well for a lawn but not for a raised vegetable bed. Choose a mix that has good nutrient and water-holding capacity. If the framing material is sitting on top of the native soil, then first dig down or rototill at least 6 inches and mix the native soil into the soil mix as you fill the bed. Called “double digging,” this is optional but will loosen the soil for deeper plant root growth. A soil test is recommended so that you can properly adjust the nutrient level and add supplemental fertilization if needed.

When mixing your own:. Add composted organic matter, such as compost and manure, to native soil. If native soil is clay, break it up into small bits by hand or with a tiller. The finished medium should consist of at least one-third native soil for nutrients and texture. Potting mix with vermiculite or perlite may be used instead of the organic matter. 
I've included some links to some OSU publications for more details: "Growing Your Own" https://extension.oregonstate.edu/catalog/pub/em-9027-growing-your-own?reference=catalog#raised-beds
Raised Bed Gardening: https://extension.oregonstate.edu/sites/extd8/files/catalog/auto/FS270.pdf
I hope this helps!
Happy Gardening!