Where should I focus my lens so I will get...

The lens should have two sets of gradations on the focusing ring. (Provided it is a manual focus camera, many autofocus lenses don't have a distance scale.) One set will be the focus distance, the other set will have the aperture 'f' numbers written beside them in a symmetrical set spaced either side of the focusin index mark.

These markings indicate what the manufacturer of the lens considers the field depth for a given lens.

If you set the infinity mark next to a particular 'f' number, the same 'f' number on the opposite side will be next to the closest point for close focus.

So if you want to ensure an object in the foreground is in focus, first focus on that object to find it's distance from the camera. Next select an aperture which allows that distance and infinity to both be positioned between the two aperture marks.

E.g. using my Pentax 50mm prime (Fixed focus) lens, set it to an aperture of f22, and place the infinity mark next to the f22 line. I find that an object only six feet away will also be in focus. At f16, it's about nine feet.

You would normally want to use the largest aperture you can to achieve this. First you normally want a shutter speed high enough to avoid camera shake for hand-held shots (a good minium is 1/focal length of the lens for 35mm cameras.). Secondly, lenses are theoretically not as sharp at smaller apertues than larger ones.

In practice lens qualtity limits the performance at the largest apertures, so the sharpest images are usually got at about the middle of the aperture range. (For an f2 lens somewhere between f5.6 and f11 will be best!)