You can find a lot of information here: rbenv on github
To list all available installation candidates:
rbenv install -l
To install a ruby version you need (for example):
rbenv install 2.0.0-p643
So that you will only use this ruby version in this specific folder and not affect anything else you can do:
rbenv local 2.0.0-p643
This will generate a .ruby-version file in that directory which will force rbenv to use this ruby version here.