Hello. I am just learning AutoCAD 2016 by myself and of course it has a lot of dificulties. As one of them, I wonder how to make lines into a complete boundary(of course it is closed).I usually use 'Extrude' to make solid, because the working piece has simple structure(several cylinders, so all I need to do was just extrude a circle.) But when I define a boundary by drawing several lines and extrude it, that doesn't give a solid but a surface made by each line(thus they are just boundary surfaces).I have to make a pillar with such boundary but don't know how to do.I tried to make a group with that lines but extrude still doesn't give me a pillar. Anyone have similar experiences before?

Try using the PressPull command.You can also try creating a region first from your groups of lines, etc. then extrude.I am a bit confused though.  First you mention trying to extrude a circle then you say you had a bunch of lines that you extruded by ended up with surfaces.  Finally you talk about creating a pillar.  I'm not really sure what it is you are trying to do.  Can you post an image or upload an actual .dwg file?If one extrudes a circle they should end up with a closed cylinder.  This could be used to create a pillar (an ornamental column).  Then one would add the base and top cap.  Is the pillar going to be tapered?
John Connor2016-01-13 11:27:39

A simple 3D pillar created using solids.>

To make a closed boundary of Lines into a single object so that when it's Extruded you get a 3D Solid, use the PEDIT command and its Join option, to make the Lines into a closed Polyline.  [If it's not clear enough what to do from the prompts, read about it in Help.]

Try using PressPull first as it will work even if the "boundary" is comprised of a couple of lines, a polyline, an arc and a spline.  The result will still be a 3Dsolid.