How to set DNS server?

If you are uncomfortable using the terminal (although I would never discourage anyone from using it and familiarizing him/her self with the terminal), you can also achieve the task using the GUI method below:

1. Open Network-Manager from the icon in the System Tray.
2. Select Edit Connections from the menu.
3. Select Wired/Wireless depending on your type of connection.
4. Double click on your particular connection.
5. Select IPv4 (you are in all probability not using IPv6).
6. Change the method to Automatic (DHCP) Addresses Only.
7. In the field provided for DNS servers, enter two DNS addresses separated by a comma. I would recommend trying Google's DNS servers at first.
8. Click Save.
9. Disconnect from your network and then reconnect to it.
10. Restart your browser to test your connection speed.

Thats it.

Edit the /etc/networking/interface file with some text editor.

Since you are not familiar with terminal, nano should be easiest ways.

Or you can use some graphical tool, like gedit, leafpad, mousepad and issue in terminal:

nano /etc/networking/interface

and paste the following:

auto lo
iface lo inet loopback
  address 127.0.0.1
  netmask 255.0.0.0

auto eth0
iface eth0 inet dhcp
    dns-nameservers 8.8.8.8
    dns-nameservers 8.8.4.4

Don't edit /etc/resolv.conf manualy. It will be reset when you restart the network.