What settings should I use to take a picture of the Supermoon tonight with my Nikon D3000?

Also would like to know how to picture stars.

You're a day late, pal. Maybe next year

Assuming it's not cloudy & grey where you are - the Moon is a sunlit object therefore the sunny 16 rule applies.

At f16 the shutter speed is 1/ISO.

So… At f16, the nearest whole stop shutter speed at ISO 200 is 1/250.

Bracket your exposures to get good details. You'll need a longish telephoto - 400mm is the bare minimum.

For stars, you either take a shortish exposure (less than 30 secs) with the ISO right up & the aperture wide open - this give you fixed star positions. OR if you want star 'trails', then take a single long exposure (or better still, stack shorter exposures of 30secs over the course of one hour or so).

Static star shot;

Star trails;