I suggest you first to use a simple model like a rectangular beam crashing on a simple rectangular wall. It is in my point of view the easiest model to start with.
Second, you have a choice to design your beam and wall on catia, solidworks, abaqus ...and export as .iges / .igs file for exploitation in LS-DYNA. Or, you can create your parts in LS-DYNA in PrePost. As material you can simply use common Iron. You will have to define:
-the beam parameters:
*deformable body
*moving body with a given velocity of course in the wall direction
*assign iron property to your beam
*define the model used to describe how your beam behaviour and parameters related to the model (yield stress, elastic constant, plastic strain rate, total strain to be reach, table defining your strain evolution ...)
-the wall parameters:
*rigid body
*fixed body
*Assign material properties (might be optional in case of rigid body)
-Do the mesh, describe de different contact to be created between wall and beam and other environmental conditions.