Firstly, your eth0 interface is not configured at all. You will need to
load the necessary driver module for the card and edit
/etc/network/interfaces to configure it.
After this is done correctly you should get it added to your routing table
automatically.
You will get an entry such as:
Destination Gateway Genmask Flags Metric Ref Use
Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0
eth0
This tells the kernel to route all packets destined for 192.168.1.0/24 to
interface eth0. This is all you need to do.