Which is used when there is a selection of choices, and you are picking one.
What is used for a description, or if you don't know what the choices are.
For your examples, it should be:
Which car are you looking at?
This is because you are probably next to your friend and have an idea of the selection of choices.
Which bicycle do you like most?
I'm assuming here that you are both in a store or looking at the same webpage, so that you know your choices.
What song did you just sing?
Here, you don't know the choices that were picked from, so you use what