If i extrude a curve with blender i obtain a cilinder with no top and bottom surface. How could I create them?
Do you mean extrude a mesh circle?
If so, there are a few ways.
1 In edit mode, select a vertex loop at one end of the cylinder and hit Shift+F key. You can then hit the F key alone and choose “Make” to make a single face. (This only removes visible edges between faces)
2 Instead of Shift+F hit Extrude>Region and S key. Then scale to the center while holding Ctrl. Then hit W key>Remove Doubles.
3 Select the vertexes in groups of 4 and hit F to make faces the way you want.
If you are using a subdivision surface, add extra vertex loops near the ends of the cylinder. (Ctrl+R, move over mesh, L mouse. Then move loop into position and L mouse to set)
Hope this helps.
You can just hit alt+m and select “at center” and it will put all the verticies at the center and automatically remove doubles, easier than trying to scale it.
Set Back and Front on and 3D off.