You can control how your clients request or book in sessions with you online.

To set this up, head over to the Settings area, then the Client Area tab.

You can turn on/off the ability for clients to book sessions online by toggling the 'Allow online Session bookings' switch:

If turned on, then further down the page you have the following configurable options which you can customise to your own preference:

If you only want to offer clients predefined session templates to choose from (for example, a set duration & price), then you would enable the 'Only allow clients to book in by choosing a Service, which uses my Session Templates list'.  
See this other article on how to setup your Session Templates

Once you've chosen your preferred settings on how session bookings are handled, your clients can start making online session bookings through either your 'Client Area Website' or through the Mobile Apps.

As an example, below is what the client will see if attempting to make a session booking through your Client Area Website...

Step 1 - the client chooses one of the Session Templates you've made available for online bookings:

Step 2 - client selects a date and time to confirm:

As the trainer, the booking will automatically be added to your Calendar schedule, and you'll receive an email notification about the booking. The client will also receive an email confirmation, as well as an option to add the event to their personal Calendar.