Your first set of instructions for Arin to complete should first to be chat with Hugh briefly in the morning.  Then make her get some coffee and a snack to perk her up a little; set her to work immediately after.
 
You need to make sure you at least have around $380 before you'll be able to afford the brown suit which is probably her best purchase, don't even bother with the white jacket and black pants combo as it's not that impressive and if Margot doesn't favour Arin, she'll simply tell her to get something more appropriate on.
 
Buy the brown suit.  IMMEDIATELY upon selecting "buy", also click onto the office and hit the coffee machine and pick some coffee, followed by the snack machine and select a snack.  You MUST do this before the $380 or so disappears from Arin's inventory - she'll still get these items even if she can't afford it if the money is still in her inventory.
 
Go from there, be talkative with Hugh, try to get as many stars as you can, and try to make sure Arin's performance is WAY above the threshold by the end of the day so that she gets a performance bonus - you'll probably need it for Tuesday. :)