Your cake needs a place to rest.  Most of the time your simple round cakes will do fine on a cake plate or cake pedestal.  However, your other sheet cakes will need a cake board.
Some options for cake boards:

hardwood pastry boards
plywood
foam core
cardboard
masonite

Once you’ve chosen a cake board, you’ll need to cover it.  You are probably [...]

I haven’t mentioned every utensil or tool you might need, but the ones that you may not already have and should get. Some (like the icing spatula) might require a trip to your local craft store, but you will use it every time.
Baking Pans: Invest in decent non-stick cake pans. I’d recommend having [...]