Before adding a new package, it is always a good idea to update your package database for the already installed packages with the up command. Then, add a new package by issuing the add PackageName command, and execute it by using PackageName in the code or in the REPL.
For example, to add 2D plotting capabilities, install the Plots package with add Plots in the Package mode by first typing ]. This installs the Plots package and all of its dependencies, building them when needed.
To make a graph of 100 random numbers between 0 and 1, execute the following commands:
using Plots plot(rand(100))
The rand(100) function is an array with 100 random numbers. This produces the following output: