定义对象的依赖注入

I'm reading into Dependency Injection and I got stuck on a bit that seems a bit vague to me.

Its used for a website application. How and where to I define every possible service? I don't even know if I'm going to use it in the particular run?

Let's say I have the following service:

1. Config
2. Cache
3. FileReader
4. Router

Where do I define all these classes/definition of the services/classes? And how do I only define them if actually needed?

Thanks in advance.