Have you got an ITVX account? If not, sign up here by using the 'Register Now' button at the foot of the page:
https://www.itv.com/watch/user/signin
How you sign in will then depend upon which device you're using but it's basically the same as signing into AB or any other site. i.e. you click/tap/select a 'Sign-in' option (if you're not already seeing it), fill in your username and password and click/tap/select 'Enter'/'Go' or whatever it's called on your device.
(If you're using a device that's not got a keyboard, entering letters and digits is usually achieved by using the up/down buttons on your remote control to select a required character and the forward/back buttons to move between characters. Then click the 'OK' button).