All these commands require sudo. Replace "packagename" or "string" with the program you're installing or searching for.
apt-get install packagename - installs a new package (but see aptitude, below)
aptitude - Curses viewer of packages installed or available. Aptitude can be used from the command-line in a similar way to apt-get, but only for some commands - install and remove being the most common. However, because aptitude keeps track of more information than apt-get does, it can be considered better at install and remove operations.
ref: ubuntu forum