I'm no expert but you will have the incoming wire connected "to" a modem "from" the modem an Ethernet connection (usually a yellow wire) that will go to your new wireless router.
You can now connect wireless or you can run an Ethernet cable directly from one of the router ports (usually 4)direct to your PC.
Routers are fairly easy to set up but you do need to setup the security for the wireless I suggest WPA2-PSK.
The router you buy should depend on your usage lots of wireless devices like tablets, phones and other PC's requires a higher end router.
250' is doable depending on where you place the router. Near a window on the shop side is better than trying to go through several walls.