I want to engrave, i.e. shape, a small toy sports car out of wood. I do not plan on doing it using chainsaws but with a few hand tools. The dimensions of the car are going to be 15cm x 12cm x 10cm. I do not know what kind of wood I should use and where can I get it. Also, I would like the wood to be light-weight and strong. If the wood is too strong or hard I will not be able to make a car out of it. Also, if it is too weak it may break very easily. Hence I want the wood to be neither very hard nor very soft.
It really depends on the level of detail you want the wood to hold. My first response would be pine/fir/spruce. Most 2" x 4" material is pine, sometimes fir or spruce. It is cheap and all are 'softwoods'. These tend to be the woods often used to make toy cars. There are lots of level of quality when talking about wood cars.
So if you want toys, the softwoods should be fine. If you are wanting to attempt more 'model' type carving you are going to want to be using hardwoods (eventually). As a beginner I'd still start with the softwoods, understanding that they have a harder time holding fine details.
When you move to hard woods (because these are more expensive so practice on cheaper woods) Basswood and Butternut are supposed to be two of the nicer woods for carving.