Usually on the right lapel.
carnation
Girls/women might wear a flower in her hair, or on her wrist. A boy/man would wear a lapel flower in the same place you would at a wedding (on the right of your chest, if he has a pocket on the right on of the suit, he puts the flower in the pocket with petals showing. Otherwise the flower is pinned on.
On the left side, always
It should be worn over his heart.
The correct way to wear a poppy is to pin it to the left collar area. If the article of clothing being worn does not have a collar, it should be worn on the left lapel area.
It is called a boutineer. It is basically the male equivalent of a corsage. If you gone to prom, it looks like the ones that guys wear.
The flower a man wears in his lapel is called a boutonniere (derived from French and means button hole in both France and England). It can be any type of flower that he chooses, but they're all called boutonnieres and are usually reserved for special occasions ie. weddings and important gatherings.
wore a red carnation in his lapel for good luck
left side lapel above notch
This is more of a fashion-oriented question. When it comes to politicians, I would say no. Politicians are to wear one lapel pin: the American Flag.
On the left and closer to the lapel than any other lower order (campaign, jubilee etc) medals.
Carnation