It all depends
Look through a few of the build threads here in this forum. You will see many many different building techniques - and they all work
For the most part, foamies have a flat wing with just a square, straight cut on the foam for the leading edge. Based on some tests Ive seen, that square edge actually works better for foamies with a flat wing airfoil than a rounded or beveled leading edge.
As far as spars - it depends on the wing span and the weight of the foamy and how you plan to fly it. For most foamies, a simple wood (spruce or other hardwood) spar is fine. Most guys tend to use carbon tubes or rods to save weight though.
Your best bet is to find a design you like in here and just do what they did. After you have built a few you will get a feel for what works best for you.
You can also tell us what your plans are and ask for recommendations - you will get lots of them and they will be mostly different AND most of them will still work just fine.
Building foamies is a pretty forgiving thing and most of the details are not all that critical.
The most important thing is to try to keep the weight down as much as possible.
Good luck and keep us posted on your ideas and progress!