Change the Form of 3D Objects that I Modeled

I’m new to Blender. I finished modeling a few 3D objects and now I have some questions about what changes I can make to them and how.

Is it possible to change the form of an object once it is modeled? For example, can I change the form of a regular plastic cup to make it look like it’s crushed?
How would I adjust the position of clothing when I’m posing my character in an active pose? For example, how can I make a dress clump up in some areas, spread out in other areas, and simply look different in each active pose?

Let me know if there are any tutorials you would suggest.

Thank you!

For example, can I change the form of a regular plastic cup to make it look like it’s crushed?
Go into edit mode and move the vertices.

Using proportional editing may help http://www.blender.org/manual/getting_started/basics/transformations/transform_control/proportional_edit.html
Using sculpt mode may help http://www.blender.org/manual/modeling/meshes/editing/sculpt_mode.html?highlight=sculpting

Thank you very much for sharing! I will use these instructions to continue working on my objects.

Also look into shape keys. These will store your different poses or shapes so you can quickly go back and forth between different ideas.

Thank you for the tip. Do you know any good tutorials that can help me properly utilize the shape keys to save poses and shapes?