If your car stereo is Bluetooth enabled and supports music over Bluetooth (AD2p?) then most any app that plays music on your phone will work. I use google play music to stream music from my library and occasionally use Jet Audio Plus. I also stream Pandora from my phone through my radio as well.
If your radio does not support Bluetooth but has a line in adapter, you can connect to the radio via your phones headphone jack and cable.
Since your question is kind of vague, that's the best I can give you right now.
Yellowjacket mentioned a couple possible solutions. I have a couple more. There are FM Transmitters that have built-in Bluetooth A2DP receivers so you can stream from the phone via Bluetooth to the FM Transmitter, which in turn modulates the audio across your choice of vacant FM radio stations on the car's radio dial...usually between two strong stations, or toward the bottom of the dial near the college stations, or near the top of the dial where channels are spaced farther apart. These transmitters often fit completely into the car power accessory outlet (AKA Cigarette Lighter jack), except for a small portion that extends out to provide the buttons for controlling what channel it is on and such.
Another option as he mentioned is if you have an auxiliary input on the car head, a 1/8" (3.5mm) stereo input. If so, there are Bluetooth to 1/8" (3.5mm) output receivers. I have one and use it to receive my streaming audio from my phone and send it into a clock/radio on my dresser so it can wake me to the music of my choice. It works fantastically well and runs for about a week before needing a charge. Here's an example but there are less expensive ones...
Another solution, with added benefits is the Motorola Droid Roadster2 (<$60) bluetooth visor-mount device. Not only an awesome hands-free speakerphone; especially if you utilize the MyMotoSpeak app, but it also features FM transmitter capabilities. Tap the FM button on the device and a voice will suggest which station to tune-in for reception.
Hope the info is helpful.