Looks like you have a few misconceptions.
Pre-poured, sterile plates will already have sterilized agar poured in them. They will arrive sterile and ready to receive tissue transfers. You'll need - at least - a still-air glovebox to work with them but you won't need a pressure cooker. You will also need a means by which to seal the plates - Parafilm or something similar.
Pre-mixed agar powder will need to be dissolved in water and sterilized prior to pouring. You'll need clean plates or vessels into which to pour this mix once sterilized in a pressure cooker or autoclave. This powdered media will contain all of the ingredients necessary to produce nutritious agar.
Agar-agar powder - on its own - is not nutritious. It is a gelling agent for nutritious media.
Here's my favorite tutorial: