Firstly thank you all for the references and tips. will need time to digest the materials mentioned here.
Let me share 2 specific challenges I face with my photos:
1) Exposure - with digital - even a slight brightness often "kills" the colors by "blowing them out". When I shoot outdoors (I live in a very sunny country) I find it very hard to set exposure - as "normal" exposure often is much to bright and colorless, if I start to underexpose - I get dark spots in the frame? Any tips on outdoor shots in bright light ?
"Understanding Exposure" by Bryan Peterson discusses metering in some depth. The first step if you are taking pictures of a scene with really bright highlights is to make sure that you are metering the primary subject and how to meter when photographing challenging scenes.
A quick summary: the way Peterson likes to do it is put the camera on manual and fill most of the frame with whatever he wants to use as his point of reference and manually set the exposure, then recompose. Alternatively one can use AE lock combined with shutter/aperture priority.
2) Portraits. Everyone talks about using a small f/stop >=2.8 to produce background blur (bokeh) and give the shot a nice affect. However I find - that f stops 2.8 and smaller - can easily produce blurry shots as the smallest movement of the subject (not to mention a group shot where people are not all the same distance from you) causes blur. I found that nothing ruins a nice portrait more then a blurry kind of picture (unless this was intended for some artistic purpose)
What is the best F/stop for portraits ? What about if you use a flash ?
The best F stop depends -- depth of field varies with subject distance. If you're close to minimum focus distance, you could need a much narrower aperture than f/2.8 to avoid "foreground blur". If you're taking a full body portrait with a 50mm lens on a crop, subject movement will not throw them out of focus (assuming you focused correctly). See online depth of field calculators.
For portraits, there is no one best F stop, but if you go with a narrow aperture, you will need a more cooperative background.
Flash can be used as necessary to provide enough light. Ideally, you choose the F stop based on depth of field preference. As you've probably observed already, for group shots you often need more depth of field, especially if the subjects aren't conveniently lined up in the plane of focus. A flash really helps for these because then you don't need to stretch the aperture to get enough light, instead you can close it to get enough depth of field.