Have you installed the
Android Auto app?
Install that, plug your phone into the head unit and you should be on your way. You might also consult the manual for your particular in-vehicle system to get the correct information about how to utilize the feature, without knowing what in-vehicle system you're working with people here can't look up the correct info to help you.
There is also an
Android Auto section here on Android Central, though you may need to register for a (free) account to be able to post in there.