If a character wears magical armour, he cannot also wear ordinary armour and, needless to say, he can only wear one set of armour. Magical shields and helms are also counted as magical armour, so you cannot have a suit of magical armour and carry a magical shield or helm unless specifically stated in the description of the magic item.
If a model has a magic shield he is allowed to wear a suit of normal armour. If a model is wearing a magic helm he is allowed to wear a suit of normal armour and carry a normal shield. If the model is wearing a suit of magic armour he is allowed to carry a normal shield.
A character that is not allowed to wear ordinary armour cannot be given magic armour/helms (unless differently specified in the Army book). A character that is not allowed to carry ordinary shields cannot be given magic shields (unless differently specified in the Army book). This includes many kinds of Wizard whose natural magical harmony would be seriously affected by armour.