Hi Ally23946,
If you go to your WP-Admin login page (example.com/wp-admin) and select Plugins, then Add New then type in Availability Calendar. Install the plugin and activate it.
It's pretty self-explanatory after that but please reply again if you are unsure and I'll try to help you out.
I have chosen to be notified for any follow-up posts via email.
Hope this helps,
Timothy Neville
Timothy Neville Website Design and Maintenance
http://www.timothyneville.co.uk