Hi Claire.  I am no expert, but I have picked up a few tips.  Most head and shoulder portraits are done using a 50mm focal length.  This gives the least distortion.  A wide angle lens such as 24mm tends to make the face balloon out.  A wide aperture will help blur the background, but not too wide, or you may get too shallow a depth of field to capture all the subjects feature in focus.  Make the best use of available light.  A hotshoe flash unit that can be angled to bounce the light off a white ceiling will give a much better tone than the on-camera flash, though if you have to use the camera's flash, you may be able to lower the strength on some cameras to give a less washed result.

Make sure you focus on the subjects eyes, unless you are being creative and want to emphasise a different feature.  Use a tripod to keep the camera nice and steady, especially if you aren't using a flash.

That's all I have.  I am sure there will be folks with more expertise who will give you some solid advice.

Andy.

I agree with what Andys pretty much said, I am only a novice so my advice is limited and only my opinion, I love portraits and have been shooting people for the past year or so. Like all photography is all about practice, practice and more practice. I tend to shoot most of my portraits using only available light and avoid flash where possible try not to be too scared of using a high ISO.  I normally use 55mm focal length, Aperture between f4 to f11, shutter speed no slower than 1/50 and have started to move away from tripods meaning higher shutter speed for hand held, I prefer hand held because its more dynamic you can move around your subject, move in or out. Don’t be afraid to get in pretty close to your model so long as there comfortable with it. I avoid using too much direction things like “smile” or “say cheese” I pretty much let the model do what they want; it makes for a more natural shot, if you want them to smile or laugh make them smile or laugh. I always do prep work even if it’s just thinking about it before the shoot, what am I trying to capture? And how can it be achieved? Composition is important, rules of 3rd, avoid cutting limbs off, focus on the eyes, make lots of mistakes, and have fun learning from them.  

Yup I know loads but its all going to depend on your lighting, artificial.... studio lights.... natural lights?  direct... indirect... high key..  low key??????  Theres so much to portrait photography that its impossible to fit it all into a reply in here.  The best tool, is your eye.  Take pics, review and see what can be changed to improve them, either F stop, angle, bounced light from your flash, white card place in various places around the subject to reflect light into shadow areas... 

I think the best thing you can do, it try one style first, but make it up as you go, quirky angles, go out in natural light for best results.  Etc.  Then put a few pics up in here.  Have a look for Lauries (capture this photography) tutorial on the homepage, its really good.  

I did a little article on practicing portrait lighting styles using a foam head from the beauty supply which you can read here: http://www.myfinepix.com.sg/article/259473/176030

I personally prefer to use a focal length of 105mm for single subject portraits.

This is a little advanced but it is still a good selection of videos on the subject of portrait lighting:

http://www.prophotolife.com/video-episode-13-beautiful-portraits-with-ju...

http://www.prophotolife.com/video-episode-14-the-classic-three-light-por...

http://www.prophotolife.com/video-episode-25-outdoor-portraits-1/

http://www.prophotolife.com/video-episode-26-outdoor-portraits-2/

http://www.prophotolife.com/video-episode-27-outdoor-portraits-3/

I recently found something that can be used as perfectly serviceable softboxes with any lights.  They are fabric storage boxes from Ikea: http://www.ikea.com/us/en/catalog/products/10193376/#/80179471

Here is a list of some every day lighting I have been using as part of a recent project which can be adapted to use for almost anything:  http://www.myfinepix.com.sg/forum/2/425876 I would suggest using higher wattage daylight balanced CFL's if you want to adapt these kinds of lights for portraits. You can also use utility lights from the home center or hardware store (ceramic sockets though not plastic) for lighting. you will have to watch ambient light carefully though as it may still affect the white balance even with daylight balanced CFL's.  They cycle at 60hz so you need to capture the full spectrum to maintain proper white balance (1/60 sec-1/120 sec...etc.) in my experience unless you purchase photography specific bulbs which can be fairly pricey compared to regular CFL's.

You can make light stands for clamp on type lights from old paint or coffee cans filled with cement and a long wooden stick.

Thanks Daz.  That's very nice of you to say.  I wouldn't say I could write a book. There are a lot of books and videos and tutorials already out there that are much better than anything I could ever put together. But I do appreciate your kind comment.

Hi

I don't think anyone has mentioned about how to get a white background which was one of your questions.

In a studio situation you Will need a little space between the sitter and the background which would need to be white or near to it.  the distance should increase for a full length picture.

The exposure on the background should be around two and one half stops more than the light on the subject. Much more than that and there is a danger of flashback creating a rim of light around the sitter which can degrades the edges.

For exterior portraits keep the face out of the sun. The shade is softer light and the further into it you go it becomes more directional, i.e. coming from the side instead of above. In the shade you need to pay attention to the white balance as the light can be affected say by overhanging trees.  shadows are normally more blue than Sunlight and if the camera is not in the same light as the subject the auto white balance may be wrong for that person.

For expressions with a static portrait you must retain eye contact. Do not hide behind the camera at the vital moment. Use a tripod, compose and focus before taking the shot while looking directly at the sitter.

Having dwelt on some technical aspects above, remember successful portraits come from person to person contact. The camera work needs to be second nature so that you can concentrate on the pictures.

For guidance on modern styles of portraits consult suitable magazines.

Best of luck.