Preparing models for injection moulding?

Hi,

How can I learn how to prepare models for injection moulding? At the moment I create a model and then send it out to someone (externally) who prepares the model/part for injection moulding?

I'm wondering what the best way of teaching myself this would be, this would save a lot of money as I currently have to pay someone else to do it, whereas I should be able to do it as I build the model.

Thanks a lot

(I use Solidworks if that's relevant)