Sign up for a chat service 3rd party, e.g.
Set this up, configure as required, follow their instructions.
Once ready, grab the widget code
Insert the widget code onto your website.
Login to your website, go to Stacks. You will then see Tracking before closing head tag and Tracking before closing body tag. These are the two areas you want to add in the widget code.
Once both blocks are in, you should have a live chat widget on your website.