command line - how i can install any program with terminal

I have installed Ubuntu 16.04 now in my computer
but the problem is the Ubuntu software can't download any apps.

So the solution is to install the any program with terminal
but I don't now how ?

Installing via terminal is usually using apt or apt-get.

First of all you need to know what package should be installed, therefor you need to know the package-name.

Assuming you want to install thunderbird you would first of all search for the package name like this

apt search thunderbird

This should output a huge list of packages - locate the proper package ayou are looking for - in our case its simple thunderbird.

Now install the package like this

sudo apt install thunderbird

Finally - i do prefer commandline to install software, but you could as well search for a solution for your base problem - the non working Ubuntu Software application. Therefor it could be helpful to start this software from terminal to get some error-output which could help finding a solution.