How to take High Res photos with a Nikon D90?

I have a Nikon D90 with a 18 to 105mm lens. I would like my photos to be less grainy and to be High Res and look professional. Is there or smething I should do in Adobe Lightroom or a setting I need to have my camera on? I try to take pics in RAW when I want high quality but the grainyness is still there.

I know how to use my camera on Manual. So if you want to explain using iso, aperture, Fstop, shutter speed. I will understand.

Without an example it's difficult to know what you mean, I assume you mean noisy images.

Shooting pictures in low light will always be noisier than those shot in bright light, if you want a low key image, shoot in bright light, in Raw, and just turn down the exposure in the Raw Converter, you'll get low noise, dark, low key, images. Lots of light is the key to low noise images.

If you ever get the chance to go on a film set or a Studio shoot you'll be surprised at how bright the lighting is, just to avoid noise.

A digital camera is just an image recording device, digital does not record light as film does, you need to get your exposure spot on to maximise data, it's like shooting with fast slide film, very little exposure tolerance.

A sure fire way to get spot on exposures is to shoot to the Histogram. Set your camera so you see the Histogram after every shot, use Aperture Priority and take a shot, look at the Histogram, what your looking for is a 'hump' of data just short, but not touching, of the right hand edge, use the EV compensation to adjust, this is the way the camera was designed to be used and why there's a dedicated control for EV compensation. You have maximised the Data.

You can do the same in Manual Mode it's just that Aperture Priority will put you strait 'into the ballpark' and you'll need fewer frames to get it right, mirror-less cameras can do this before you take the shot.

Shoot to get the maximum data in camera, post process for effects is the 'rule of thumb'. The creativity happens in post.

There are times, of course, when you can't control the lighting, more modern sensors are better at low noise, notably the Sony sensor in the Nikon D7000, Pentax K5 and K-01 and the Sony Nex-7, they have noticeably lower noise, but still work better when there's plenty of light.

Noise - what you call grain, will be worse as you increase ISO. Always start out with the lowest ISO - in the case of the D90, that would be ISO 200.

And if you are cropping the bejesus out of your photos, they are going to show noise no matter what ISO your are using (or camera for that matter).

If you want your images to "look professional" then learn photography properly.

You will not learn it on here by asking basic questions.