I would like to know where I have to install Juju with Maas to deploy Wordpress first ? and Openstack later ?
Is it in my maas server ? or Is it in one of my node ?
You can start simple, install juju in the same server where you have MaaS.
What you want now is to get the latest juju from the ppa:
apt-get install python-software-properties
sudo add-apt-repository ppa:juju/stable
sudo apt-get update && sudo apt-get install juju-core
Now the juju configuration:
Follow the documentation about the configuration here