“To make your car stereo louder, you need to buy an aftermarket amplifier and possibly a subwoofer.
You can purchase amps and subs online, from big box stores like Walmart or Best Buy, or from a local car audio shop. And if you don’t know how to install it, you can pay someone from one of the aforementioned establishments to do it for you—even if you didn’t buy the equipment there (usually, at least).