Hi Maryam –
Raised beds dry out a little faster than your average garden so we usually recommend adding an organic compost to fill at least half of the bed. Compost is heavier than most potting soils and will retain moisture longer — it’s also cheaper than potting soil. Fill the remainder of your bed with a good organic potting soil, preferably one with some organic nutrients already added. I’ve included a few links that should help.
https://www.planetnatural.com/raised-bed-soil/
https://www.planetnatural.com/garden-soil/
https://www.planetnatural.com/compost-sewage/
https://www.planetnatural.com/product/organic-garden-compost/
https://www.planetnatural.com/product/organic-compost/
https://www.planetnatural.com/product/roots-organic-soil/
https://www.planetnatural.com/product/happy-frog-potting-soil/