What lens would be best?
I'm looking into buying my first lens for my nikon d3100. I use my kit lens 24/7 basically. I'm going to need one with a built in motor. I'm looking into a 35mm, i do everything from portraits to landscape to action and more, i'm not picky on brand names but i'd like it to atleast be decent. I'm going to also be using this lens on a nikon d40 and maybe soon a nikon d5100
Any Nikkor AF-S lens will work with all those cameras
List of available Nikkor lenses
On a 35mm film camera or full-frame DSLR a 50mm lens is considered a "normal" lens since its angle of view approximates that of the human eye. Since your Nikon has a 1.5x "crop factor" a 35mm lens on it will have the angle of view of a 52.2mm lens on a 35mm film camera or full-frame DSLR. So the 35mm lens on your Nikon would be a "normal" lens and would allow you to photograph a scene close to how you saw it with your own eyes. It will not be a good choice for portraits or landscapes.
On a 35mm film camera or full-frame DSLR a focal length of between 85mm to 105mm is considered an ideal portrait focal length. A 50mm lens on your Nikon with its 1.5x "crop factor will have the angle of view of a 75mm lens on a 35mm film camera or full-frame DSLR. A 50mm lens on your Nikon would be a good choice for portraits.
Another good choice for portraits with your Nikon would be the AF-S Micro Nikkor 60mm f2.8G ED which will have the angle of view of a 90mm lens on a 35mm film camera or full-frame DSLR. Plus, since its a macro lens (Nikon calls it "micro" for reasons unknown to me) you can make great close-ups of bees, ants and other insects.
The 18mm end of your 18-55mm lens is a good choice for landscapes unless you want to spend the money for the AF-S DX Nikkor 10-24mm f3.5-4.5G ED which would definitely give you those wide sweeping landscapes you may desire.
So buy the 35mm lens if your goal is to photograph scenes close to how you see them, buy the 50mm or 60mm for portraits and either use the 18mm end of your zoom for landscapes or buy the Nikkor 10-24mm wide angle zoom.
When you say you also photograph action well that's a little vague. If you mean action sports like soccer or football or basketball or motor sports then you will need a longer zoom like the Nikkor AF-S 70-200mm or longer.
Happy shopping.
35mm f/1.8 is a great one with 52.5mm after crop.
But bear in mind that it's a fixed focal length lens
The lens you already have would achieve the same as what you're looking for. I would get a 50 f1.4 if you want a faster one
I will recommend nikon too