How do I know which ISO and aperture?

You are walking down the street, with your camera on a shoulder strap around your shoulder.
The last shot you did was indoor, so your settings won´t do any good outside.

Suddenly, nature, people, world and everything gives you a picture in your head. You quickly draw your camera, turn it on - but wait, what ISO and settings should I choose?

Is this something I learn?
Is it like driving? When do I know to brake before a turn?
Do I just have to learn by mistakes? (I haven´t done that driving!)

Generally its better to keep the ISO as low as possible to avoid noise.

The aperture essentially lets in more light, but this also determines the amount of depth of field. The wider the aperture (the lower the f stop) the narrower the depth of field.

So in bright sunlight it is difficult to get shallow DOF because the image will be overexposed. For this reason I recommend a variable ND filter on the lens.

So, go outside and:

1) Lower the ISO
2) Establish the DOF you want by setting the aperture
3) Dial in the ND filter to achieve correct exposure

With practice this is like driving a car.

As I thought. I just have to learn by doing.
It´s a lot to learn, but I have learned a lot the last months. Thanks to you guys!