It might depend on the area, but ask your line manager.
Basically fill a form in and your manager countersigns it and it's sent off and the season ticket will arrive in the post (usually very quick too).
AFAIK you cannot apply before your start date as basically you're not in the system.
It says you are in Manchester, it might be worth stumping up the extra cash for a System One County card for buses and trains plus you can use the ticket for your own leisure use - that's if you commute within the County Card area.