It’s all about the lighting. (OK, it’s not, but a good portion is.)
If you want a nice head shot or head and shoulders shot, there is a quick and easy way to get a better-than-average picture – even professional quality if you know what you’re doing.
It’s called a window.
Turn off any artificial lighting in your room. Place your subject near to a window, with her body at a 45 degree to the window. She could fact out the window, or be directly looking at you. (You should position pretty much against the window or wall as well.
Now, use a moderate telephoto lens (an 85mm, 105mm, or 135mm would be ideal). If you have a 35-70 zoom, use it at the 70mm setting. Otherwise, use your 50mm lens. Do not use wide angles if you want flattering portraits.
To compose, fill your frame. Don’t be shy, get close. Set your focus on the eyes. I personally like portraits shot at very open aperatures (f2.8 or lower.) This gives the picture a low depth of field and leads the viewer directly to the eyes (provided you’re correctly focused.)
Play around with her positioning to the window a bit. Look at how the shadows fall around her eyes. You don’t really want shadows cutting into her eyes, so pay attention to that.
Otherwise, if you want good portraits outdoors, cloudy days tend to work better than sunny ones. If you’re going to try to take good pictures in the sun, wait til late evening (or wake up early), to catch the sun while it’s still fairly low in the sky. Winter should be a decent time for portraits in Paris, as the winter sun stays low in the sun pretty much the whole day round, so you’ll have plenty of time to take those shots. You just have to be able to recognize nice lighting when you see it. Be careful, though, as most subjects will squint if the sun is slightly overpowering.