To access the dedicated server for Race07 or GTR-EVO (same thing really) in Steam open the tools tab in Steam and launch Race07 Dedicated server.

Next you need to port forward some TCP and UDP port traffic from your router to your server PC to get an internet server going 

You will need to be able to log into the control panel for your router and set up port forward rules that will point to the IP address of the machine on you local LAN running the server. 

Ususally you create a custom rule for each of these putting in the ip of the server running steam eg 192.168.1.4, 10.0.0.4 If the PC has a dynamicly assigned ip address be aware that it can sometimes change, this would happen if you have PC's coming in and out of the network that might grab the IP that you assign in the router.

Set port forward rules for each of these ports

TCP+UDP 48942 – 48957 (all ports in this range)

UDP 26900-26907 (all ports in this range)

For the port forwarding if you do not know how to then download a manual for your router, they usually explain how to do it in there.
On your local computer server if you have a firewall enabled you will need to allow the ports for the traffic to come thru or turn off the firewall.