Cytuj:
Don't get resistances until you have about 1500 HP.
Then get 0-100 points in the resistance of your choice, based on physical/magical balance of their team; 100 being all one type, 0 being an even split.
Then get 1 point in the resistance of your choice for every 75 health you get
Flat Pen - Get extra armor equal to their flat pen divided by the complement of their %Pen.
%Pen - Get less armor equal to 100/(%Pen)-100
Obviously, items don't come in continuous quantities, so just aim to be somewhere near this rubric.
Also, different champions are different, so if you have shields or heals, get some more resistances, and if they have % Health damage, get more resistances.