Hmmm …. to answer with the same briefness …
write some code, that accesses the alfresco repository via one of the various APIs (SOAP or REST webservices, FTP, CMIS, CIFS, Email)
For a more detailed answer, I would need more details 🙂
But before you do this, maybe you should have a look at the product presentations on the Alfresco Website, the "Getting started with DM" Guide for FTP, CIFS, Email and at the Developer Guide in the Wiki for developement API examples. (use the "Browse for categoriy" menu link to fine your way best).
Regards, Norgan