Selecting subjects to photograph

The aim is to produce pleasing well balanced photos.
Do not put the object of your interest in the center of the photo.
With multiple subjects, pull all elements together, e.g. have every one looking in the same direction.
Keep your backgrounds simple.
Choose your subject's best point of view. Move in close.
On very dark days use a flash for nearby subjects.

What filters can I use to enhance B&W work?

Generally the net effect of using colored filters will be that objects the same color as the filter will become lighter in your print (darker on the negative), while those of opposite on the color to the filter will become darker.
The more saturated the filter, the more exaggerated the effect.
Yellow filters darken blues e.g. the sky.
Red filters dramatically darken greens - good for separating red flowers from green leaves, they will also darken blue skies a lot.

Polarizing filters, will darken blue sky so the clouds stand out more, without changing color balance of other things and are useful for reducing glare from polarized light e.g. on water, leaves, metal, etc.
Neutral density filters decrease the brightness of the scene overall without changing tonal relationships within the scene, this allows the use of slower shutter speeds or wider apertures.
If you use a separate light meter, you will need to adjust your exposures for the filter factor.

What are guide numbers?

With an electronic flash, the guide number for a given film speed ISO is an indicator of the power, i.e. "light output"of the flash.
The bigger the guide number, more powerful the flash.
The guide number is the product of subject distance multiplied by the f-stop.
It can be converted from one ISO to another by the formula GN2=GN1(sqrt(ISO2/ISO1)).