The player first resolves any actions or rules that must happen at the start of the turn.

Some warriors will have special actions they have to perform at the start of the turn (Orcs and Goblins have to test for Animosity, for example). Such rules are not common, and their details will be clearly stated in the relevant Warhammer army book. If you don't have any actions to perform during the Start of Turn sub- phase, it's worth using it as a tide break (or breathing space) between one turn and the next where you can remove stray casualties, errant dice and all the other bits of gaming detritus that builds up on the battlefield as the game goes on.