The lavaan package is available on CRAN. Therefore, to install lavaan, simply start up R, and type in the R console:

install.packages("lavaan", dependencies = TRUE)

You can check if the installation was succesful by typing

This is lavaan 0.6-18
lavaan is FREE software! Please report any bugs.

A startup message will be displayed showing the version number (always report this in your papers), and a reminder that this is free software. If you see this message, you are ready to start.