I’d use ASA 100, or slower, simply because you’re gonna need a lengthy exposure to get decent trails. And this gives you the added bonus of a finer grain. I recommend stopping the lens down pretty much all the way. Use whatever focal length frames your shot pleasantly.
Star trails will begin to appear about about one minute, but they don’t really look like much until you reach at least a fifteen minute exposure. One-half hour to a couple hours is better, or even four hours. (obviously, though, if you’re taking four hour exposures, you ain’t gonna get to take very many.) If you point your camera at the North Star, you can easily calculate the degrees of arc the star trails will describe for a given length of exposure. One hour is 1/24th of a full revolution; thus, the star trail will imprint an arc of 15 degrees.
The moon will begin creating trails much sooner since it’s closer and also has it’s own motion relative to earth. However, the moon won’t ever look like anything other than a smear. You’ll probably want to avoid capturing it entirely. You’ll also probably want to avoid shooting even star trail pictures on nights with a bright moon. It tends to wash everything else out of the picture since you need a pretty lengthy exposure to get decent star trails.
Another thing you can try, is using your flash manually off camera to highlight objects such as trees, or rocks, in the foreground. Also, try tracing foreground objects with a flashlight - maybe even with a colored filter on it. Just trace the objects once or twice and leave the flashlight off for the rest of the exposure. These techniques work better than you might initially suspect. You need not shorten your exposure time to use these techniques.
Another thing you might consider, is making a flat black card to hold in front of the lens while you’re tripping the shutter. Cameras can shake a little bit when the shutter snaps open or closed. But if you have a flat black object in front of the lens, there’s no image to show that jitter. The longer focal length lens you use, the more important this becomes.