From Ubuntu Terminal screen (Applications > Accessories > Terminal), type:
taufanlubis@toshiba:~$ sudo apt-get install xchat
[sudo] password for taufanlubis:
Reading package lists… Done
Building dependency tree
Reading state information… Done
The following extra packages will be installed:
The following NEW packages will be installed:
0 upgraded, 2 newly installed, 0 to remove and 2 not upgraded.
Need to get 1358kB of archives.
After unpacking 3928kB of additional disk space will be used.
Do you want to continue [Y/n]? y