I am new at cake decorating and this is my first naughty cake. I have a pan to bake the cake, but I want to put some kind of cream filling in it. Do I inject it or is there other ways.

You can make a hollow from the back after baking; or use a turkey baster to squeeze it in like filling a cupcake (or Twinkie); or make a tunnel-of-fudge-type cake with the filling baked in.