if you have a tripod (which you should for night landscape), keep the ISO at 100. aperture really depends. optimized sharpness would be around f8-f11, although you may have to adjust depending on available light, the DOF required etc. There's no exact rule as it really depends on what you're shooting, where you're shooting, what lens and body etc.
take a shot and adjust from there. don't be afraid even if the shot turns out bad even only after you realise it at home on the computer. at least you learn from it and know how you can prevent the same mistakes again.
we have a sub-forum with lots of info regarding taking photo at night. may i suggest u go through them 1st, than try shooting, after that post ur photo that u face problem with and ask for help (pls specify which area u facing problem).
Just play with the settings, usually from F8- F22 @ ISO 100. rigid tripod is a must because you deal with slow shutter speed. Dont use Spot metering while doing nightscape photography. bring also mosquito repellant.)