If there are choices available when you are hired on, the most senior (oldest in the class) usually has the choice. When the choices run out, pilots are assigned fleet and seat. Lately, with the MD 11 new hires, only new hires with heavy experience are usually offered the MD spots.
Even if they tell you what seat you can expect, everything is subject to the needs of service. Most times nothing changes, but sometimes the seat / fleet does change. Just part of the fun working for Uncle Brown.
Good Luck,
Freig8t Dog.