You will need two exposures: one long exposure for the landscape and one short exposure for the stars. Shoot the landscape at the lowest-possible ISO and try to limit the ISO for the sky, too, while retaining acceptable background colour - this is particularly critical for night shots (somewhat counterintuitively) as noise (particularly unwanted chroma noise) is most apparent in dark, shadowy areas, such as night skies. You may even want to make a third, longer, low-ISO exposure, purely for the night sky (which often contains blues, greens and purples, rather than being black), then heal/clone out the short star trails in post-processing. Also, while you will probably be shooting stopped down to f/11, or even narrower, for the landscape, you can shoot the sky exposures wide open, as it is all at infinity - this will help increase the brightness of the star exposure and decrease the length of star trails (and the amount of PP required) for the night sky exposure.
Prior to merging the two (or three) layers, you will firstly have to boost the brightness of the stars in Photoshop, as the short exposure will most likely be quite dim. At the same time, you don't want to increase the brightness of the dark, surrounding night sky, as this would also amplify the noise and give you an unrealistically bright sky. You can do this in curves - adjust the curve so that the brighter areas (stars) are much brighter, while the dark areas (sky) remain dark). If you took a third exposure for the night sky, you will also have to combine it with the star exposure. After you have cloned/healed out the star trails in the night sky exposure, drag the layer containing the pinpoint stars above the night sky layer, then apply a layer mask to the star layer. Copy the image showing the pinpoint stars and paste it into the layer mask - this way the bright points (stars) will be visible, but the dark areas (the underexposed night sky) will be transparent, allowing the well-exposed night sky underneath to be seen. You can improve this by applying Levels to the layer mask, so that any illuminated pixel (i.e. the haloes surrounding the stars, as well as the stars themselves) is fully opaque, while all dark areas are as transparent as they can possibly be.
With that done, all that's left to do is combine it with the landscape, which I'm sure you know how to do already!