Well not knowing what you are building I will try to give you a little general info about it. The standard board used in house construction is the 2x4, there is a similar product in steel construction that is referred to as a "C" it is about the same size as it's lumber counterpart and fits into a track top and bottom of your wall. Go no less than 20 ga on this. this product can be used to make roof trusses as well. For metal roofing use the 26ga galvalume. it is a little more expensive, but in the long run you will be glad you used it. As for the layout of wall studs, most tape measures have a diamond shaped mark on them about every 19 ( more or less ) inches, this is for metal carpentry. Make things easier on your self by purchasing some good self-tapping screws to erect your studs and trusses, these will keep you from having to pre-drill holes . It is not as difficult as you think, I have built with both steel and wood and I would choose steel every time.
15 yrs carpentry and construction experience