this site and shapeways in general is only for printing of digitally created parts. if you are familiar with 3d software- either engineering based or artistically based- the you would have to "create" the part in the computer. next is sinding the part digitally to shapeways to print.
now here is where it gets tricky. there are many ways of digitally creating a printable peace.
- if it is an engineered part the it is relatively simple to design it straight in the computer.
- if it is a sculpture or organic shape you have two basic options
-- use a sculpting program like z-brush, mud-box, or sculptris
-- use a 3d scanner to scan the object.
this is your basic process in outline form. to further complicate things there are engineering software that can sculpt, and sculpting programs with some of the capabilities of engineering software.
the last this that i can say is that if all else fails you could ask one of us, designers, to help you out. but that is if you are so inclined