Nice start. When I started, I studied the default X-Plane aircraft carefully. It will show you things like how they do windows. I make my windows one way. So they are greyed out from the outside a bit through the glass_NML.png file and no scratches or dirt on the inside like the default planes.
Just a tip in case you haven't discovered this yet (sorry if I'm stating something you know), but after you finish an object be sure to make it have triangles. Easy way is to select the object and press SHIFT-CONTROL T (or menu is Surface/Triangulate). In X-Plane, everything has to be a triangle which is the meaning of the "TRIS" numbers in the .obj files (i.e., "TRIS" = triangles with the location of each triangle's three vertices). . .
If I'm building something with small parts, I'll often cut squares into triangles as I go by selecting two vertices and press SHIFT_CONTROL L (for line). This impacts how the object reflects the light so you will sometimes see your object with unusual black patches if a vertice is off line . . .
Feel free to PM me if you're really stuck. I'm no expert, but I'm using AC3D most every morning at 2:30 am when I wake up for a few hours!
Best,
Steve