I suspect you're talking about Ethernet, and you want to put the cable modem/dsl in another building?
The usual approach is to use some kind of fiber transceiver for the optical links at each end. The fiber transceiver has an AUI connector on it. Plug the transceiver onto an AUI hub (such as a DELNI) and also plug a 10baseT transceiver onto it. Some better-quality 10baseT hubs have an AUI port for uplinking.
Terminating the fiber (putting connectors on the ends) is hard work. Also you'll need to jacket the fiber so it doesn't get damaged. Depending on how much money you have to spend, you may want to consider a radio or laser link instead.