Tips for night time shooting. Nikon d90 with 50mm prime lens?

I need tips.

my pictures always come out a little noisy and the light always a little too yellow. What can i do about these two problems?

also, I know that if you have a small f/stop number such as 1.8 then you can have a faster shutter speed. What would be the ideal iso?

Bed time

Yellow is caused by not setting your white balance to incandescent

Noise is caused by high ISO settings.

My guess is that you have both the ISO and white balance set to "auto"

The D90 is a fine camera. Learning to use it as a manual camera will enhance the quality of the images it produces

The ideal ISO will be the one that allows you to shoot at shutter speeds of 1/250th second or faster

Forget using f.18 for night time work unless you are hand holding the camera for 'people' shots.

For night landscapes, trails etc you need a tripod.

Shoot manual, have the ISO at 100 or 200 & the aperture about f11-f16 for good depth of field. Then expose for about 10 secs or so (city lights) & see how th image is. If its too dark, expose for longer, if it too light, expose fro less time.

Orange colour is from white balance being off. City scapes are often a real mixture of colour temperatures in the lights & its impossible for the camera to render each colour accurately. Best you can do is shoot in raw and then tweak WB in post.

Edit: I did think you meant night landscapes when reading your question, but if you are looking to take low light shots 'on the fly' (clubs etc), then the above is not relevant. For 'people' shots, you WILL havbe to have the aperture wide open & the ISO cranked up just to get enough light. Even them it may not be enough - you might have to think about getting flash to add some.

The ideal ISO would be the lowest you can get away with. High ISO = Noise. An f/1.8 would be fine for shots of people at night, or objects close by - but you might still need a steady hand (or tripod) if you are not using flash. If it's really dark - then you are still going to need a long exposure and a tripod.

There are no ideal settings for a camera because it will depend on the available light. For night landscapes you'd be better with f/8, a tripod, and a long exposure.

The yellowness is because of the street lights. Either do a manual white balance, or shoot RAW - and process later.

Your D90 is very similar to my D300 in image quality. It may even be better. See these samples and click on the thumbnails to examine them individually. You will see that I have put the exif data right under the image for most. If I didn't, just click on "Nikon D300" over on the right and it will show. You can see what settings I used there. A tripod or other stabilizing device (other than VR) will also help.

If the yellow pictures are indoors, set the WB to incandescent. If they are outdoors, you can try the same setting, but street lights are often some form of mercury vapor and the WB is crazy. I use RAW and AWB and I can fix it later, if necessary. Often it is fine.