I have been given a project to link a PIC microcontroler to a system and then back to the another system. that is a PIC between two systems. How could I do that. and also how do i Create an sms gatway?

 

You mention USB in the subject line. It is both a poor choice for a computer to computer link assuming it would be possible to have both a slave port and and master port on the same device, and an incredibly difficult thing to implement. You probably have no idea of the complexity of such an undertaking.

I think the best choices might be asyncronous serial communications with an RS-485 physical layer.

 

check the USB PIC18fXXXX of Microchip, they have usefull information in their site

 

Do you mean to use USB or a parallel port? They are very different.

Here is the Wikipedia article on SMS gateways - http://en.wikipedia.org/wiki/SMS_gateways.