The easiest way to get ggplot2 is to install the whole tidyverse and also learn how to Install gglot2 and tidyverse:

Go to R console and type
install.package("tidyverse")   ## Downloads and installs the package

Alternatively, install just ggplot2:

install.packages(“ggplot2”)

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To check package installed or not

In R console just type 
library("ggplot2")      ## loads the installed package 
library("tidyverse")