You just keep completing the tasks as they appear in your to do list. You can't just focus on any one storyline, as they intersect (i.e. Mary's intertwines with Becca's intertwines with Clara's intertwines with Danny's, etc etc etc). That is, you can't complete individual character's storylines one after the other. There are also times when a task may appear on the list that you cannot complete until another character's tasks catch up.
Also, there's playthrough videos on YouTube - the most popular are by MoonSpawn:
You must be registered to see the links
The start of the 6th video in the playlist is at the point of the task "Find some food in the fridge" which starts that salad scene. However, if you haven't peeked on Mary in the bathroom yet, you probably won't activate it yet. The 5th video shows you some of the actions you should take before that task opens up.
Note, some of the task orders in the game have altered since MoonSpawn's videos were posted - however, they will be satisfactory in most cases.