Yarrick's pic is a nice, simple example of the orky aesthetic. Just like their vehicles, ork terrain should look haphazardly cobbled together, but with a clear function in mind. Crude plating, shoddy reinforcements, jagged crenelations, and numerous glyph plates are always safe bets.
As for materials, foamboard/foamcore is great for blocking out your basic structures, and cardboard and/or plasticard will be invaluable for adding details. Other materials that I've found useful, which can be bought cheaply or scrounged: wire, plastic/wire mesh, popsicle sticks, toothpicks, corrugated cardboard (carefully tear off one side and you've got yourself passable corrugated metal), cardboard/steel cans and tins, disposable chopsticks (cheap to buy packs, free to grab a few extra if you eat out

), pen parts (tubes, springs, etc.), sprue bits (ground up for rubble or left in short lengths for the odd brace) and plastic gears and gubbins from old electronics, to name a few.
When constructing larger structures, it helps to plan the structure somewhat lopsided. The more unbalanced and precarious the projections, the better. If the materials can support the shape of the model, itself, chances are a mek could and would build it. If you want to make enough to cover a table, I'd suggest you start small - a few barricades, a small building or two - to give yourself a chance to feel around the aesthetic and practice construction techniques. Once you're a little more confident and you've built up your building supplies a bit more, start tackling the big, characterful projects like the mek's workshop and the holding pens.