Widest is 18mm, cropped sensor, Nikon D5100
Cool so this is how you calculate the exposure using the "500 rule" to avoid star trails. Of course if star trails are your goal you can throw most of this out the window. But if you want any chance of capturing the milkyway and the most stars possible you will need to adhere to these rules.
You take the width of the lens and multiple by 1.5 to account for the crop factor of the sensor. (if using a full frame sensor camera you dont need to do this and will be able to take longer exposures without star trail)
so 18x1.5 = 27 This means your sensor and lens combo really equate to 27mm not 18mm.
Then simply divide 500 by the crop factoedr width so 500/27 =18.5 This is your maximum exposure time at your most wide aperture you can use without getting star trails. I would just round your exposure time up to 20 seconds.
You are losing about 1 full stop of light at f3.5 so typically your ISO starting point would be 3200 but in this case you would probably want to start at 6400 which will produce more grain and color noise but shouldnt be bad unless you are blowing up the picture or cropping in post.
Im not real familar with Nikon but if your camera has it shoot in raw not jpeg.
-Set you color ballance to either tungsten or 3200k
-Of course you will need a tripod.
-If your camera has it turn off low light noise reduction. Its better to handle that in post with something like Lightroom.
-Turn mirror lockup on or use live view mode to lock up the mirror to avoid any camera shake.
-Use the 2 sec drive mode timer on the camera again to avoid any shake from pushing the shutter release button. or use a remote cable release.
-Shoot in manual mode and turn off Auto focus on the lens if you have it.
-You will need to find true infinity of your lens. You cannot trust the infinity mark on the lens. It may be accurate but you need to check it first. Do this by going out on a nice clear day and find the sharpest focus point of an object that it far away as possible. If your camera has it use live view magnification to do this. Note where the focus ring is metered. If its dead on then great you can use that when doing astro. If its off you can either use what ever reading is showing on the lens as infinity or use a piece of gaffers tape to hold the lens at that focal point or better yet just to give you a place to mark a line with a pen on the lens for true infinity. You will need this to set the manual focus in the dark especially at f3.5. trust me when I say you will not be able to accuratley focus the lens without it in such dark conditions.
If your camera has it use the histogram to determine proper exposure. You cannot trust the rear display in these conditions. If you are unfamiliar with understanding the histogram let me know and I will be happy to explain it.
Now dont forget that these kind of images break a lot of general rules in Photography and you will need to do some post processing. Light room is best for this but you could probably use a lot of other programs to some degree.
Also something I didnt note above if shooting in Raw format you will need a program like lightroom that recognizes raw format to process the image. Otherwise you will need to use jpeg and you might still get some good shots but you will be severely limited in what you can do and color will most likely be off quite a bit and you may need to experiment with other WB settings.
There are a lot of post process techniques that are needed for astro but here are some of the main ones.
-turn the highlights to the lowest setting possible then bring up the shadows to the point it looks best.
-bump your saturation and vibrance setting to the max this will look bad but it allows you to then adjust the temperature setting to the point where you have an even amount of blue and magenta in the image then adjust the tintt until you just start to have a bit of green in the image but not too much. your white balance has now been set and you can reduce the saturation and vibrance settings until the image looks natural or is pleasing to you.
-finaly you eill need to map your tone curve. This is somewhat complicated to explain without know what post processing program you are using but in general you want to slightly clip the blacks then pull the darks down just a bit and pull up the whites just a bit to get a very subtle S curve. Of course if you dont have a program that does all this you might have to just mess with the exposer and contrast sliders to try to get the proper tones.
I know this is alot of info to throw at you but you should understand why your images might not appear as expected just using the camera settings above. If you are interested Im more than happy to take you and anyone else who wants to come out on a shoot and will show you everything I know. If you dont have software to process the image we can do that at my place then if you like it and want to keep going you can get the Lightroom CC and PhotoShop CC cloud software for just $10 a month and have more image processing power than you could ever want.