Do you already have an aux input? If so, all you need is the patch cable. If you have a cassette player, you can use a cassette adapter. If you have neither of these things and don't want anything installed then you need to go with an fm transmitter.
These things are hit or miss and whether they work or not can depend on many variables, such as what kind of car you have, where you live/drive, and the availability of clear frequencies. For some, the really inexpensive ones work just fine. For me, not so much so. I've tried several before finally settling on the DLO Transpod/Transdock. It was pricey but it was the best for me and my Honda Civic in San Diego. They run anywhere from $75-100, so before you go that route, you might want to go ahead and get some less expensive ones and try them out. Just make sure you get them from a place that has a decent return policy. I would probably advise against doing anything mail order just because of the hassle of returning them by mail then waiting to credit your account. Unless you don't mind that hassle.
I can't think of anything that will allow you to choose songs on it that isn't something that needs to be installed. There are some with remotes (DLO has one), but they are fairly rudimentary, only allowing skipping tracks forward and back. I mainly rely on playlists on my Touch, and I find it easier to select what I want using the touch screen than the click wheel of the other iPod models.
I have an 8GB Ipod Touch and was looking to use it in my car but dont want to bother with buying all this stuff that needs to get installed....Can anyone suggest a good car adapter to use with my IPOD? I want to be able to choose songs with it.