How can I connect a laptop and xbox to a router using only 1 line with out losing strength

Connect the xbox and the laptop to the ports on the router. One of the ports will be marked WAN, connect that into your DSL/Cable modem. Follow the router manufacturers instructions for setting up the router if you have not already done so.

You only have so much bandwidth, the more concurrent users you have, the less you have available. Unless you have a slow connection you probably won't notice too much of a speed loss with the xbox going.

Most routers have a built in switch, usually 4 ports. You can connect both devices to those ports. A switch shares the bandwidth more efficiently than a hub so there is little degradation from concurrent users.