Welcome to the Workshop community. We're really pleased to have you join us.
First question is - what are you planning to grow? That will obviously affect what you should use in your new garden box.
Let me also direct you to some recent discussions about filling raised garden beds that you might find very useful:
Welcome to the Workshop community @Symofam2. I can advise you on the soil required for the garden box.
You will need 0.8 cubic meters of soil or 800 Liters if buying in bags. The type of soil you decide on will depend on what you intend to grow in the box. For a good all-round product, I would advise Osmocote Professional 50l Premium Potting Mix as it is suitable for a variety of different plants including herbs and vegetables. It also contains a wetting agent and a controlled-release fertiliser to aid in plant growth. If you would like to create a more nourishing mix, you can add additional organic material such as Anl 25l Cow Manure.
Please let me know if you need any further advice on soil mixes or have other questions.