If you have Internet connection you can download the soft that you want to and install it.
See the APT-GET guide.
If you shutdown the computer, this programs that you have been installed will not be the next time that you turn it. But there is a way to make a partition in the HDD or something like that (I never did that). See this question where are four answers about that.
If you don't have Internet connection there is another way to install soft. You have to download it from another computer to the USB and then install it to the off-line computer. See this answer.