The ticket collection options available to you will depend on your route and the stations you are travelling to and from, but the most common are:
E-Tickets
Delivered electronically to your email
Download the QR Code to your mobile wallet and use it to scan through the barriers.
Heads Up! - Not available on all routes, e.g. when using multiple train providers
Ticket on Departure
Collect a paper ticket at any UK station with a ticket machine or ticket office.
Available for all routes
To collect, insert any payment card (doesn't need to be the original payment card used for the transaction) into the ticket machine - it WILL NOT be charged, it is purely for ID purposes and the booking reference from your confirmation to print.
For more information on booking your trains have a look at your Rail Booking Guide!