I m shooting a NASCAR race with a Canon EOS 5D with a EF 24-105 lens. What setting should I use?

Depends upon a lot of factors. Where are you going to be? Is this a night or daytime race? What's the weather going to be like? Do you want to stop the action so that even the rotation of the tires has stopped? Or, do you want to show a sense of speed by using a longer exposure to blur the background as you pan with the cars as they pass in front of you.

My guess is that since you're only using a 24-105, that you're not going to be in a good spot for photos, and you're likely to be in the stands. If so. just experiment with fast and slow shutter speeds. The cars are doing upwards of 200 laps, so you'll have plenty of opportunities to review your shots and make adjustments as you go along.

There is no one best setting. You choose the aperture to get the area that you want in focus or to get the shutter speed that you want for the desired effect that you want (e.g. slow for panning; fast to stop motion). Once you have the aperture, then you'll either allow the camera to choose the shutter speed for you in aperture priority, or use a specific shutter speed in manual mode. Using manual mode is often preferred so that your exposure doesn't change as you point the camera at white or black car.

I would definitely suggest that you set the AF system to be off at the shutter button and on at the rear AF button. You will find that if you use the shutter button to activate the focusing system, that the camera will likely shift focusing or often hunt each time you press the button. With the rear focusing button, you focus one time, and then you can compose the shot and wait for peek action all without having to keep the shutter button press half way.

I would also put the camera into continuous AF and continuous shooting mode.

Use the lowest ISO that you can. Don't automatically choose 400 unless you need to get a faster shutter speed or use a smaller aperture to increase your depth of field.

Experiment with panning. Start off with 1/30th shutter speed, increasing the shots by a stop to as much as 1 second. You'll need to keep an eye on the aperture information. If it starts to blink, then that's the camera saying that it can't chose an aperture so that the image is properly exposed. At that point either lower your ISO, use a faster shutter speed, or put a polarizer or neutral density filter on your lens to cut the light down.

I strongly suggest that you go to 500px.com and do a search for NASCAR or auto racing. Click on an image, and then click on the details tab on the right. This will show you the camera, lens, and exposure used. Doing this will help you with ideas of what shots you'd like to try to capture, and what settings are good for those types of shots.

Set AF to servo and select a central focus point
If you want to blurr the motion of the cars use a shutter speed 1/60th or so.
If you want to freeze the cars then use a shutter speed of 1/1000th or faster
If you want to pan the cars use shutter speed of 1/100th or so.
Set ISO and f-stop to suit the light level.

If all that is gobbledy **** set P mode, ISO 400, and pray.

Depends on technique you use and direction of cars. It they go towards you the shutter speed can be longer, but if perpendicularly then you have to increase shutter speed. You can also make panoraming where the shutter speed is relatively long (e.g. 1/30), but you move camera with subject during making photo. And you can't predict weather and lighting. So to be short: you have to experiment. There's no other option.

Since you asked, set ISO to 1600 then Auto everything else.

If you don't know, you have no business owning an EOS 5D.

I'd also suggest that 105mm isn't long enough if you're one of those fussy people who doesn't want a car bumper removed from your kidneys.

the one that takes a video