How to focus manually using a non af-s lens

I am a new owner of a 3100, it really takes great pictures. Yesterday I found I could not use my old af lens which worked on my d70. I was told by the Niconites that it would not work and to focus the lens manually or get a AF-S lens. I am a little short of change since buy the 3100. the lens I am using is a Tamoron 28-200 mm and it ran fine in the d70 in the AF mode. I shot a few shots and manually focus and the subjects were out of focus. I was using a spot focus so I switch to matrix and that seemed to work better, at least they were is focus, but not all shots. I think I am the right path. What other settings should I use to maintain focus and or am I on it and is that the best it going to get until I get a AF-S lens. Can some one clear me on what I can do in the meantime.
Kas

It seems that you misunderstood some basic technical terms - "matrix" and "spot" are the light metering modes (the way your camera calculates the elements of exposure, as to avoid over/underexposing the photo - the sensor, in combination with other electronic components measures the intensity of light flowing through the lens, so to speak).
Focusing is a different thing - you turn your camera to the certain object, and try to make that object look "sharp", while the areas in front and behind the object remain relatively "blurred". To manually focus, you have to rotate the "focus ring", until you either estimate that the focus point has been "achieved" or rely on a "green dot" ("focus confirmation"), if it works with your combo (camera body+lens).

Please be more specific about your Tamron (tell us the EXACT, full designation of the lens). It is probably made in Nikon's AF-D fashion ("screwdriver AF") meaning that autofocus works only with cameras (bodies) equipped with motor drive. Is it this lens, by any chance: http://goo.gl/0LsCTu ?

As DraganDL said you have to manually turn the focus barrel ring to manually focus the AF-S lens. However, you will have to switch the auto focu OFF on the camera. At least, I do on mine. I have a switch on the left camera body for AF / MF.

Now, if you are having issues achieving focus, that is a different matter entirely. Most of these DSLR's come with a matte focus screen with no split prism to give you a focus indication inside the viewfinder. If you use the viewfinder to compose your shot you would look in the lower left corner of the finder screen and look for the green focus confirmation LED to light up.

If you compose using live view LCD screen I am not sure what indicates focus other than seeing a sharp image on the screen. I had issues with this and I use my viewfinder like I did with all of my film cameras.

I also suggest you stay in Matrix metering mode. Set your camera AF settings to Auto Area or Center Area, use center point auto focus instead of dynamic or multi point tracking.

It's a D3100, that does not have a motor drive, so it is not necessary to do anything (no switching from AF to MF) except turning the focus ring. By using the term "spot", our colleague has maybe referred to "dot" (1, or 9, or 11 dots, I guess, that can be chosen as focus points).

If you are careful, it is possible to use the focusing system of your camera to check (verify) focus. It is a little tricky but if you practice, you can get pretty good.
First, you would have to use a single point focus (I don't have this model so you'll have to do your homework and dig in the user manual to find how to set it up). Then, you half depress the shutter while focusing and you should see the rangefinder focusing lights in the bottom left of your viewfinder. It consist in two arrow with a round symbol in the middle. The idea is to focus until the arrows disappear and the round light turns on in the middle. This means that the spot where you focused should (note that I wrote should) be in focus.

The other really foolproof is to use a tripod and liveView, using the + keys to zoom in the liveView mode and then acquire focus and then shoot. As you see, this is not a feasible option when you are hand holding the camera, but it works and you are sure of your focus.

I find it sad that the person that sold you the 3100 did not mention this to you BEFORE selling it.

the d70 has an internal focusing motor, the d3100 not. I dont know this lens, but if your lens has no internal motor but is an AF, then it relys on the cameras internally built drive motor for af. The D3000 and 5000 cameras series dont have these internal motors.