Viewfinder problems?

Okay so i have a nikon D3100 i'm a beginner so if i mess up then thats why so i apologize in advance!
Okay so i use my view finder and my pictures look so good through the view finder but once they've been taken they look unrelatable… Two different scenes /lightings/images look horrible…
What can i do to make what i see through my lens actually what i see?
I've been playing around with apertures (manual) and iso sensitivity but i just want a better stream of photos. So thank you so much!

If you are using manual, messing with the apertures is probably messing up the light you need for a correct exposure. Use what the meter tells you to, and it should be fine.

You need to understand the fundamental of photography. This can take up to four semesters of photography at a community college.

For now, spend some time on this site, learning how to balance ISO, shutter speed and lens aperture

There's NOTHING wrong with your camera

http://camerasim.com/camera-simulator/

What you see through the viewfinder has NOTHING to do with how the final shot will look. The viewfinder is just a look through the lens which is not related to how the camera is set.

Do you not know how to read and use the lightmeter in the camera? That is showing you if your exposure settings are going to cause an underexposed or overexposed shot. You have adjustments for shutter speed, aperture, and ISO to correct the problem. WHICH to adjust is where your basic photography knowledge has to come into play, as well as what your goal is with the photo. Depending on the photo in question, adjusting one parameter may be far better than adjusting another.

I suggest you not use the Manual mode until your skill is better. Use one of the semi auto modes such as shutter or aperture priority.

Here is an excellent book that would be of great help to you:
http://www.amazon.com/...81740502X/