Nikon D90 nightime settings?

I have recently been taking photos at night, and experiencing different modes ( M, A, S, P ) however i'm still not getting the hang of the whole nightime photographing! I don't have a tripod but will get one soon when i can, so if anyone can help me on how to get clear photos? Muchly appreciated!

You already know the answer - a tripod.
You can't take long exposures without some means of keeping the camera very still - in most cases, that means a tripod.
On top of that, you may want to use an external shutter release or a timer.
Go to your local library - you'll find heaps of books on photography, and they always have a chapter on night time photography, too.

What do you mean by "photos at night"?

If you mean of landscapes, the sky, etc at night, you'll need a tripod. Those sorts of shots require long exposures, which in turn means you'll need a stable surface. Seeing as how the outside world lacks them, you'd need a tripod to get good shots.

If you mean of events at night (like your friends hanging out in a parking lot), the easiest way is to switch to Aperture Priority (A) and shoot wide open. Compensate by bumping up the ISO if you still get blurry images. If you still get blurry images, you'll also need a tripod as well. If you still get dark images, push down your EV meter to -1 stop to get brighter pictures.

I use and recommend this site for low-light/night exposures:
http://www.calculator.org/...osure.aspx Using the Scene 'Distant view of city skyline or floodlit buildings' and ISO 200 resulted in these pictures:
100mm @ f11 with a 30 second exposure.
200mm @ f11 with a 30 second exposure.

Needless to say, my camera was on a tripod and I used a remote release to hold the shutter open for the long exposure. My camera was in Manual with the shutter speed set at "B".

Without a tripod you'll need to find a solid surface to sit your camera on when making long exposures.