First Install bridge-utils
run below command in teminal

sudo aptitude install bridge-utils

Now you need to stop the network by running below command

sudo /etc/init.d/networking stop

Now edit interfaces files

sudo gedit /etc/network/interfaces

add this lines to interfaces file

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto eth1
iface eth1 inet manual

auto br0
iface br0 inet dhcp
bridge_ports eth0 eth1

Noe restart the network

sudo /etc/init.d/networking start