Most Phones Have Bluetooth Now, but Some Don't
The exact process of pairing a phone to a car audio system varies depending on the specific phone and the way the infotainment or audio system is set up. Most of these steps will translate in one way or another regardless of what type of phone you have, and the car you drive, but the first step in any case is to make sure that you're working with the right tools.
Before you get started, you'll want to make sure that your phone has Bluetooth. You can go ahead and turn it on at this point, unless it was already on. Also make note of where the "make phone discoverable" and "search for devices" options are, since you'll need those in a little while. (Most phones will only remain discoverable for a couple of minutes, though, so you don't actually have to activate that yet.)