How to use Manual-Focus on a DSLR?

I'm planning to buy a Nikon D3100 a week later and just wanted to know how to use the Manual Focus.
Em a beginner.

See:
http://www.dummies.com/how-to/content/how-to-focus-a-nikon-dslr-manually.html

Go to the user manual. Link below

Look into the viewfinder while turning the focus ring. The image will become sharp when it's in focus (and a circle will pop up at the status line in the viewfinder).

1. Ensure that the viewfinder dioptre adjustment is set for your eyesight
2. Switch camera and/or lens to manual focus mode
3. Look through viewfinder and rotate the lens focus ring until the image is in focus.
4. Release the shutter

To be honest manual focus is not really needed.
Use autofocus:
1. Set AF point to centre point only
2. Point camera so centre AF point is where focus is required
3. Press focus lock or half press and hold the shutter release (depends on camera)
4 Recompose the shot
5 Release the shutter

Read the manual. I'm a professional photographer with 45 years under my belt making money with a camera. But when I entered the digital camera era from a film camera I read the cotton picking manual 3 times and still carry it with me for any little use items I might need to reference to. Also you need to invest in a few good books. I recommend the author Scott Kelby. His books take you step by step in clear easy to undertand language, plus you have acces to his web site that expands on each book he writes to give you a better understanding of this craft.

There's a toggle switch on the lens which says AF and MF for auto-focus and manual focus respectively.

As others stated, put the camera in MF mode by either setting a switch on the side of the lens or in a camera menu item if your lens doesn't have a switch.

Turn the focusing ring until the image looks sharp. Your viewfinder diopter control must be properly set. This is something you do when you first get your camera.

What others didn't say:
To fine tune the focus, place your focus point on the object you want to focus on and use the focus indicator light that you will see in the lower left part of the viewfinder. When it lights up, your image is in focus.

If doing closeup or photography of objects that don't move, you can use live view. Live view has the capability of zooming in so you can do critical focus. I often use this technique when doing macro photography and product photography.

Better yet, use auto focus as BriaR suggests. This is the technique many photographers use. If you use MF, you'll miss those fleeting shots.