For skulpted works (best for organic and free-form style character and object designs) starting with a "clay sphere" is also common. In terms of app for the iPa

For skulpted works (best for organic and free-form style character and object designs) starting with a "clay sphere" is also common. In terms of app for the iPad: Nomad Skulpt is well favoured there. Other popular approaches to designing shapes are using live addition/subtraction, Nomad supports that as well, but it's not as intentional as say 3D Studio Max (for Windows) - where animating those interactions is purposely included as their point of difference. There's also solid geometry modelling, this is intended for items that would be manufactured, for that SolidWorks is common. Then finally you have software like Maya which lets you take all manner of approaches, including a few I haven't listed here (such as scripted or procedural modelling.) The disadvantage here is that the learning curve is a mountain. And if you like a bit of history, PovRay is still around where you feed it text mark-up to produce a render.