Dear Jean,
I agree with Michael. To begin with every possible variable at its most challenging is an enormous task. If you begin your work with at least the lighting variable under your control, all will be much more fluid.
As for clothing, I think overalls are very difficult, since they tend to be very stiff and fail to move with the underlying form. Closer fitting blue jeans can still fit the desire for "informal" and will be more manageable to paint. A shirt in a very soft jersey, even a t-shirt type material is better than a stiffer denim shirt or oxford-type shirt or bulky sweater. (It's what I chose for my own self portrait in this example:
http://forum.portraitartist.com/show...5&pagenumber=1)
Like the light, I think it's better to choose clothing that won't fight you either. Leave the formal clothing and more complicated setting for a future portrait.