You can call a function or object from a package directly. This uses part of the package without changing your R environment.
package_name::function_name(...)
If you'll use a function or object many times, you may want to load the package instead. This lets you call the package contents like you would any other function/object.
library(package_name)
function_name(...)
Leave the library
line in your code file; you'll need to run it once, each time you start a fresh R session.