When I want to play against my brother in LFS(we have the same IP address), I don't know what to do. What should I do to play on LAN? Help, please!

This thread: http://www.lfsforum.net/showthread.php?t=9572 should help.

Quote from bmwracer67 :

we have the same IP address

If you have the same IP, they should never able to talk.

Make sure they have an IP in the same subnet (For EXAMPLE give one 192.168.1.1/255.255.255.0 and the other 192.168.1.2/255.255.255.0), make any firewall exceptions required, then go into LFS, click Multiplayer > Start New Game, select LOCAL from the top, give it a name, etc., make sure Dedicated host = no and click go.

Now goto the other PC start LFS, goto Multiplayer > Join a specific game, select LOCAL NETWORK and type the IP address of the computer hosting the game, and change the port / password if necessary. Click go.

Take a look at the thread thisnameistaken linked for firewalls. The rest of it focuses on actual internet hosts. Also beaware that LFS only accepts LAN IPs in the correct range.