Hello! I’ve started with Unity and with making my own game. But at the moment I have a big beginner problem and I don’t know where I can search for the solution. I’ve got some environment, but for the gaming story I need for example an oven in a family house. Is it possible for example, to paint a picture with a program like corel and to take it into my game? I’m sorry, it’s an absolutely beginnerproblem. 
Yes, but do you want a 2D picture of an oven, or do you want an actual 3D oven? In the latter case you can’t do that with a 2D painting program like Corel. You’d need to use a modeling app like Blender or Cheetah3D.
My first game shall be 2D. So the oven has to be 2D. It shall be an old oven like in the old fairy tales.
Then you shall have success with making your sprites in a paint program. 