If you haven't already, you can sign up for a free plan. Getting started is painless and consists of two steps.
I am not a power "live chat" user by any means (first-timer actually), but I found the process pretty simple to complete.
Once you have enabled the module and configured it properly as well as have installed and configured the IM client on your PC, you can browse to your website and see the chat window in the browser allowing customers and visitors to interact with you. It's all very, very simple.
You can style the chat window using various themes provided by Olark.
The easiest way to install the Olark Live Chat Module on your website is via the Orchard Dashboard. Just browse the Orchard Gallery like you do for other modules and install/enable it from there. You can easily do a search for olark to find and install the module with little effort.
Here it is on the Orchard Gallery.