Please follow these few steps to star working with bibliometrix:

1) Download and install the most recent version of R (https://cran.r-project.org/)

2) Download and install the most recent version of Rstudio (https://www.rstudio.com)

3) Open Rstudio and then, in the console window, digit:

   install.packages("bibliometrix")   # to install the bibliometrix stable version from CRAN

    or 

   install.packages("devtools")         # to install the bibliometrix most recent version from GITHUB

   devtools::install_github("massimoaria/bibliometrix")

Stable version is published on R CRAN Package Repository.